This section has no limits, well only limited by your imagination, there are so many things you can do, especially in the custom department, things like chopping, sectioning, channelling, lengthening, shortening, raking ... and would make this page into a book which is just to long for this general tips section.
Cutting out doors, boots or whatever, like anything can be done several ways, I'll show you two of these.
The first way is with the back (yes the back) of a x-acto knife, carefully scribe along the body lines you want to cut out, take your time and don't rush it as you'll slip and put a big mark down your model and/or cut yourself. Corners need extra patience as these can get tricky, but scribe along these lines and after a bit of scribing the panels should come out nicely, then just a light sand around them and your ready for hinging. The second method is using cotton; this method is probably faster but a lot harder. The trick is to either start at the bottom or drill a small hole in the corner then with a see saw motion carefully work your way along the lines you want cut. Be very careful not to go to fast or get carried away, working to fast will make you cut non straight lines and will also heat the cotton up to much making it break. If you can get someone to hold the model while you cut it then it would be great, other wise wrap the cotton around your leg (if your sitting) or around any solid object while holding the other end tight and move the model up and down. Again take your time and watch what your doing as the cotton can have a mind of its own.
This is probably one of the best things you can do to a model because it moves, where all other detailing is static. You'll need one rod and one copper pipe (sizes to come later), depending on what you're hinging, be it a boot or door they will be different lengths. So I'll explain the door method and you should be able to do the same for trunks etc but with a wider piece. I hope to add some pictures here later, as they would explain it better. Since I have no idea what model you'll be using this on its a bit hard to mention lengths etc so use your best judgement. The hinge, like all hinges works best off flat areas. Some doors are totally curved so a smaller hinge would be needed. Cut your pipe (carefully, don't squash the ends) the width of your flat panel, now insert your smaller rod inside, leaving about an inch either side, now bend at the pipe 90 degrees so you have a u shape. Now get a pen or thicker pipe and roll your rod the way the rod is pointing, you want at least a semi circle. Do this for both side, now align with the first bend (at the pipe) make another bend 90 degrees upwards so it looks like a speed hump in the same direction the last roll was made, you may have to change this curve to suit your panels. Now cut two more small (about 3mm) bits of pipe, add these to the legs and cut off any access rod, (leave a couple mm if you wish). Tape your door onto your model; place the hinge on the inside of the door with either the legs on the door or legs on the firewall (they open different directions, legs on door and the door opens inwards and vice versa). Use tape and tape or mark and hold the hinges in place, remove outside tape and try open/close door, if there is no binding, glue the legs and large pipe being careful not to get any glue in the large pipe, once dry check again for binding. The legs should be glued but the rod should be able to slide out of them, you may need to sand the inner edge of your door depending on which way you hinged it. You may also need to bend the curve to suit. You now have a hinged door. For doors its best to make some inner doors, this also hides the hinges but gives a better looking door, don't forget to add door/boot lips to the door way.
One of my favourite tools is a automotive points file, I use this for all small sanding or door handles, body lines, emblems etc but you can use any small (fine) files or whatever, if you use sandpaper be sure to use a soft rubber as a sanding block. One thing with emblems and lines they can come back, yep ghost lines, when you prime your model with thin paint these lines etc can reappear. So prime a few times and gently sand out using a fine sandpaper (600 grit +), some lines etc maybe need filling. Automotive polyester bog is handy for this, as it doesn't sink as some modelling fillers do. Whatever you use fill it one day, sand it the next, prime it the next, do it over time so it can have a chance to sink if its going to, be careful not to get it in places you don't want it. Also sanding the area with 320grit first will help the bondo stick better.
When customising we need smaller windows, some windows can be cut down, place some tape over your window and carefully scribe across it until you cut through, don't rush it or press hard as the screen will craze. To make side windows, which are usually flat, you can use anything clear. The plastic that comes with business shirts is a good one or the plastic that comes with kid's toys. To make tinted windows, use exposed negatives (film negatives) this can be brown or clear black depending on the film type (black and white film negatives are clear black). You can also use real car tint, check out your local glass tinter for left overs etc, or photo shop for film negatives. Using thicker clear plastic is good for making door glass, if you hinge the door leave a gap at the top of the door so you can drop the glass in later. Make your window appear half down (or up depending how you view it) as it gives more detail.
This is very general; lowering can be anything from turning the springs upside down, to cutting some off the springs or using no springs at all and gluing your wheels to the body (model has no underneath).
Like anything there is no right or wrong way, and are also many ways to do the same thing, here is one way of doing this.
First off you need to find some big wheels to fill these tubs. Cut some plastic sheet so it goes around the wheel 1.4 times and is about 1/4 wider then the width of the wheel, carefully work the plastic into a round ball. The tighter the better but be careful not to actually bend it. I find rolling it into a ball, and place into an old film canister (remove film first) you can pour boiling water in it (be careful not to get it on yourself). Pour water out and leave to cool, the plastic should hold the shape of the curve, if not roll it some more until it does. Once you have the curve you need to cut the length in half (one each side), the reason I got you to do so long is as I have no idea how high or low your tubs will be, but this should be enough for the lowest of stances. Carefully work out where these tubs will sit in your model, careful cut these areas out and test fit every cut you make. Cutting too much can make more work for yourself, which usually isn't nice work.
You should now have a big gap in your model base. Trim the curved strips to the desired body shape (do one and trace onto the other) then you can glue these into place. Take the diff (rear end) you'll use and mount to your wheels, sit your wheels in place, now you have the height for your center section, again I have no idea what rear end set-up you'll be using so I'll just mention one way. If you make a u shape so the middle bit is the length of your center gap and the sides of the u the front and back you should be right for most diff set up. Glue this in place once desired height is reached, make some sides for the tub sections and your done, leave for a day or so for the glue to hold then sand and fill to suit.
Tubs can be great but if you place tubs in a car which you've hinged the boot (trunk) on you'll need to make a new boot floor as the models one wouldn't have been made for it, you'll also have to blend your interior panels to suit.
Wash all parts before painting as they have some gel or something from factory that can ruin your paint job, make sure all is sanded properly and remember the smallest mark will show up, always a good idea to prime first as this will show up all your defects.
Whether you use an airbrush or spray cans, spray in light even coats, spray in a dust free area on a sunny dry day.
Use red plastic to make taillights you can make any shape or colour you please, if mounted from behind looks recessed.
Most kits come with head lights but if you loose one or want to make some use clear plastic, get a wire brush or scriber and make lines across your plastic. With a small amount of heat you can shape this plastic to suit (warning to much heat and it will melt and if dripped on you will cause a lot of pain. Do this out side, as the fumes are deadly). Apart from the lenses to make a more realistic headlight you can add a small dot in the headlight bucket so as if a bulb was in there, the more scribing you do the less that's visible, play with some rubbish stuff first to get the desired result.
Carefully cut away the clear edges from decals, these yellow over time and look horrible, use warm soapy water and a dry towel to damp off excess water as this too can stain.