Properly reassembling a machine before returning it to a customer is an important step of the repair process.
This is a list of things to do it right, and things not to do wrong.
One common mistake we see when people mess with their MacBooks is called the 'longscrew'. Some machines have screws that are meant to be screwed in a metal hole that is soldered on the PCB. The PCB has no hole in it however, so the screw needs to be the right length. If it is too long, as it is being screwed, it will dig into the PCB, and potentially damage some tracks, possibly in the internal layers if it gets deep. So make sure you never put a screw that's too long in these kind of holes.
Generally speaking, don't assume that all screws inside the machine are the same, pay a lot of attention to what you unscrew and what goes where so you can properly reassemble things the same way.
TO DO: Pictures of each machine type with description of which screw type goes where
I find that the ifixit guides for logic board replacement provide good documentation of all screw lengths for logic board to top case mounting. Just as an interim resource until someone else possibly does some specifically for this Wiki. I usually Google something like “ifixit+logic board+Macbook model size type and year. For example ifixit logic board MacBook Pro 13” retina 2015” yields this as the best result https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Display+Early+2015+Logic+Board+Replacement/45134
When working on a device for the first few times, (when one is not yet familiar with it), reassembly can be greatly aided by carefully documenting the disassembly, whether utilising a series of photographs, a time-lapse, or even a video. This way, nothing will be missed out or incorrectly placed, for example cable routing. A record of which screw came out of where can help avoid the longscrew damage described above; a photograph of the screw next to the hole from which it was removed followed by a photograph of that screw being placed in a section of a screw arrangement grid is sufficient for this if a recording is impractical.
Self tapping screws into plasticEdit
Some devices use self tapping screws tapped straight into plastic holes. While reassembling these devices, take care that you fit the screw properly into the old thread so you will not break the plastic or create a new thread. This is especially important for older devices where the plastic may have become brittle. A way to avoid this is by slowly turning the screw CCW (so as if you were to unscrew the screw) until you hear the screw click and fall down the end of the plastic thread. Then rotate it carefully into the thread as you normally would. Be sure to keep the screw at the proper angle while doing this. If there is friction, chances are you're creating a new thread. If you think it's creating a new thread, you can undo the screw and retry until there is hardly any friction. If you know it involves brittle plastic, be sure to not over-tighten the screw at the end.