the thing about pre paint mods is that its supposed to help make the job easier in getting rid of seam lines etc. and to make assembly processes smoother before and after you paint.

not all gunpla are the same, so finding a way to do a pre-paint mod is up to you depending on your kit.

 i hope that this can help give you an idea on how to do one yourself.

take this SD Sazabi for an example:

as you can see, there is a huge seam line cutting through the whole head. if you want to fix this the inner parts would be required to be painted before working on the seam which will lead to masking and other tedious work. unless, you mod the head before-hand so that you can work on these two parts separately.

the inner part connects to the armor using the common plug and socket connection. wherein both parts are held in place which doesn't allow any movement. so what i did i cut the lower part of the socket connection which ended up like this:

it is supposed to have a square like socket, but after the mod it is now more like a ''U''.

so instead of having to insert the two parts, you can now just easily slide the inner-head through the bottom like so:

as i said before, not all modes have the same design, which require different ways of modding. but once you get the hang of pre-paint mods, getting rid of seam lines would be easier.

now with the pre-paint mod applied, i can start working on that seam line without having to worry about painting problems.

now pre-paint mods can be applied to other areas such as the arms, legs, body, etc. of your model. you just need to observe the design of your kit and get creative, 

goodluck!