Corner blocks used in trim around doors and windows serve two purposes. They give an elegance and classic architectural look to the trimwork, especially if the blocks have an ornate design. Secondly, they make the trim far easier to install because you use all straight cuts without having to deal with angles. This plan assumes you already have your door jambs installed.
Things You'll Need
Two corner blocks for the top of the door, at least as wide as the door trim
Two corner blocks for the bottom of the door, at least as wide as the door trim and as tall as the floor moulding
Drill with pilot bit
Set your level vertically along the inner edge of your doorway, where the trim is going to go. Draw a pencil mark extending that inner-edge line up a few inches above the top of the door. Do the same for the horizontal top edge of the doorway, so you've got two sides of a square just off the upper corner of the doorway. Repeat for the other upper corner.
Set one top corner block in place at the corner of the door, within the two perpendicular lines. Drill pilot holes an inch in from each corner of the block. Nail it in with trim nails, setting the heads with your nailset. Repeat for the other top corner block.
Measure the horizontal space between the two blocks, and use your saw to cut a piece of straight trim at that length. The end cuts on the trim should be straight across the width of the board, with no angles.
Set the trim in place between the two blocks, lining up the bottom edge of the trim with the bottom edges of both blocks. If the blocks are taller than the trim is wide, let the blocks rise above the trim on top, but keep the bottoms even. Drill pairs of pilot holes every six inches in the trim and nail it to the wall with your hammer and nailset.
Repeat the process for the bottom corners of the door, using corner blocks that are tall enough to completely span the height of the floor moulding.
Measure and cut the two vertical trim pieces in the same manner as you did the horizontal piece, placing them between the top corner pieces and the bottom corner pieces. Fill all nail holes with wood putty.
Because corner blocks often contain ornate carved designs, it's a good idea to paint or finish them before you put them up, rather than trying to do it when they're on the wall.
Eye protection should be worn when cutting or drilling wood.