I think you are not referring to simply trimming off the excess lace which comes with all new systems....you mean the system is bigger than your thin/bald area. This can be tricky for a newbie, but as long as you take care, you can do it easily enough.
The best /safest way to do it is to get a styrofoam head-form and pin your system to it, lace-side up [hair will be in contact with the styro-head]..... T-pins are best as they`ll prevent the system moving...don`t over-stretch the lace when you pin it down...just slight ` tension` is enough.
Make a template [using kitchen-film and sellotape] of the area of the head, to be covered.... make sure it`s pretty ` solid` / rigid. It will obviously be concave in shape [as it`s made to the contour of your head-shape] This is hard to describe in words, without pics, but once made, sit it on a flat surface [table] then press down firmly from on top so that it ` pops` kind of ` inside-out` ---in other words, the perimeter now, instead of being in contact with the table, is now facing the ceiling and what was , before, the ` dome` [highest point] is now in contact with the table !....... take it and sit it onto the lace so that the bit that is / was sitting on your front hairline, is now sitting directly at the front hairline of the system [ lined-up with the front].......pin it down in a few places, all around the edge.......obviously, as the template is smaller than the system, you`ll have ` excess` lace all around the outer edge of the template. You trim off the ` excess` lace all the way around by using a sharp blade ..... one of those Hobby-knives is ideal...DON`T cut down into/through the lace, but rather draw the blade ` across` the lace surface in a sawing-motion...if you cut down into the lace, you`ll slice through hairs underneath which you actually require/need to keep.......all you want is to slice gently across the fabric [lace] and it will part from the rest of the system------- you should never cut around the front hairline area , as this will ruin the look of the front..hence you always pin the template to the very front to avoid cutting the very front.....it`s ok to trim at the ` side` [near the front] if that be the shape of the template...that would represent the temple area/s of your head, but by pinning the template on exactly ` at` the very front-edge, you ` keep` the natural look which is built-in at the factory.
The lace is relatively delicate [yet hard-wearing too !] so it`s very easy to trim it but always just draw your blade ` across` the surface of the lace ONLY ....never push the blade down through it.
Hopefully I`ve explained it in a way you can follow !