Measure the size of each wall in the room to get an exact area. There are five different cuts that will need to be made to do an entire room. There are left and right inside corners and left and right outside corners. There is also a splice cut to join two pieces for a long wall. Measure the angles of each wall with an angle protractor and divide by two. This will give the exact measurement to set the saw to.
Place the piece of bed molding on the miter saw upside down to ensure the correct cut is made. Line up the saw 45 degrees to the left for an inside right cut. The bulk of the molding should be to the left of the blade. Hold on to the molding, turn on the saw and pull down on the handle to cut the piece. Line up the saw 45 degrees to the right and repeat the same process for an inside left cut.
For both right and left outside cuts, simply turn the molding over and follow the procedure for inside cuts. The saw should still be at 45 degrees or close to it depending on the measurements of the angle of the walls.
For splice cuts set up the saw at 45 degrees in either direction. Make a cut at one end of a piece of molding and then leave the setup exactly the same. Make a second cut on an end of a second piece of molding. There will be a perfect splice every time as long as the angle of the miter saw blade stays the same.