Depends on the type of plant.
Stem plants with leaves growing from a central stem, just nip/cut off above a leaf/leafpair and plant the bit you've just nipped off in the substrate - most will grow that way, avoid crushing the stem when you nip it off, if you think you have, take the cutting and make a clean cut with a knife above the damage. If you plant a crushed stem, it will often rot before the new roots have a chance to anchor it. The stem you nipped off will usually grow more than one replacement shoots - i.e. it becomes bushier.
Rosette plants, first, try to get a plant that will fit your tank. Swords, Crypts etc., come in all sizes, find one that matches. If you do need to trim them, break of the entire leaf and take it out of the tank, they'll just rot - you can't propogate them that way. Same goes with grassy plants.
Root stock plants like Anubias sp, cut the root stock up as required.
There are endless strategies depending on the actual plant, bt that is general advice.