If its lace you`re trimming, some guys have used a razor or an exacto knife.
Cut from the underside, that is.

Myself, I`ve never done it with a razor, always used scissors.
Lace is a funny material, its stretchy and when I put a razor to it, it seemed to want to snag the lace and stretch is a little before cutting it.
Scissors hold it in place good while they cut. If you`re going to do it with scissors, use small sharp ones.

When you make cuts, hold the hair lace side up, insert the lower blade under the lace, and only open them (the scissors) up just enough to fit the lace in and cut. Keep the blade thats under the lace right up against the lace and dont open it, only open the one thats on your side of the lace.
This will prevent hairs from getting in there and being trimmed, leaving short little stubbles that you dont want.

Try and avoid cutting the very front hairline, other than trimming the extra lace thats there. Trim that lace close to the hairs but leave about 1/8th inch of the extra lace.
We do this for a couple reasons, one so that there very frontal hairs have plenty of base to anchor them and keep them from becoming flyaway and unmanageable hairs, and also, as the base ages the lace in front will fray a little, so you have a bit of extra for trimming off and making it like new again.

Keep in mind that the hairs in the front are put in more artistically, they are staggered and thinner in density at the very front with a gradual increase in density as it goes back.

Cutting into that can ruin the effect, so try and avoid too much cutting of the base in that area. If you do have to, be careful about cutting hairs while you cut the base.

If you do need to trim the front and theres no getting around it due to your preference for the shape of your front hairline, do it just like you would trim the rest of the base.
Carefully.
There will be a few hairs that get cut, and there`ll be little stubs of them sticking out, you`ll have to remove them carefully.
Basically, you`ll need to "rebuild" the artistic work that was done, and to do so, you`ll need to remove not only the little stubbles that will will be trimmed inadvertently, and are unavoidable, but also some of the other hairs to restore the staggered look.

Get the base and hairs wet, then use tweezers or something to get a hold on the short hairs and pull them slowly until the knot comes untied and keep pulling slowly until the hair is pulled out.
Do the same with some of the other hairs in a staggered and random fashion to recreate the natural hairline.

You want to pull the hairs out so they come out knot and all. You dont want to pull too fast and break the hair, or you`ll be left with the knot, or some short hairs that look out of place.
It may be necessary to to apply some conditioner to the knots to help loosen them up.

Be prepared to spend an hour or so on the whole job, that is, if you will also be reshaping the front.
The rest should set you back less than a half hour or so.