assuming you saw is square and true, thus, so are your joints, do the following (assuming its painted trim, and not finished wood)
1) make the joint. shim the top and/or bottom of each piece to get the corner to be tight top and bottom. glue and nail the baseboards.
2) inject latex caulking or drywall compound (if you have a sausage gun) into the top edge of the baseboard. put enough in that its protruding a little bit from the joint.
3) using a flexible 1 inch wide putty knife, place the 1 inch edge against the wall, and the blade vertical. pull gently along the top of the trim until the filler starts to build up on the blade. wipe off with a sponge and repeat. do the whole piece this way. if you do it carefully enough, you will have a perfect little square edged bead of filler that will dry to look like part of the trim.
4) touch up the trim paint on to the bead of filler.