Measure from a starting corner out along one wall, using a tape measure. Select a piece of wall molding that's longer than the measurement. Set it on your miter saw, with its back against the saw fence, so it's in the position it will be against the wall.
Set a piece of molding that's longer than the wall on which it's going on a miter saw, standing along the back guide of the saw in the same way it will sit along the wall. Put the starting end of the molding under the blade, with a little room beyond it.
Swivel the blade to a 45-degree miter, pointing inward, toward the long side of the molding. Cut it.
Lay the trim face down and measure on the back of it, from the cut edge that will touch the corner, out along the length of the trim. Mark the other at the distance of the far corner.
Miter the other end of the trim as you did the first end, again turning the blade to a 45-degree angle inward. This will be the opposite swivel as before.
Repeat the process for each piece of wall trim. Once they're all cut, set them in place with the mitered corners butting against each other. Secure them with a finish nail gun, shooting nails every foot along the top and bottom edges of each piece.