If trimming a limb large enough to have a branch collar, you want to make the final cut right at the end of the collar.
If a small tree, and trimming on the end of a branch (not at the trunk), cut on the outside (away from the trunk) of a bud, making an angled cut. If you want to cut the entire branch off, some small trees will have a branch collar visible, you want to cut there. Others that don't, well, it's hard to describe where to cut, but after you've pruned several hundred/thousand in the last six years, you just know.
If the branch is small (less than 1/2"-3/4"), it probably won't have a branch collar, cut slightly out from the trunk. At a young age, they won't be as affected by not-quite-proper cuts as they will be at a mor mature age.
I used to work at a tree farm. If it is young then we always cut it flush if it was tiny. If it was of any size then cut where the collar would be if it doesn't have one. One note, if you are using hand shears then cut with the blade on the side that will be left so the anvil (or the dull side) will bruise the limb that the tree is losing. Never really saved a tree like that but I figure every little bit helps.