Much of the molding that is installed on the inside or outside of a modern house can be cut with a circular saw. This is not the only way to complete the task, but cutting with a circular saw is straightforward, the tool is comparatively inexpensive and the results make for an attractive finish. It's important to follow guidelines for safely operating a circular saw to cut molding.
Nail a sturdy plank between two sturdy saw horses to create a stable work area.
Test the saw to see that it is working properly, the blade is properly installed and the teeth are sharp. A fine-toothed, cross-cut blade is the best choice, but many woodworkers get by with a well-made, general-purpose, carbide-tipped blade.
Set the depth of the saw blade so it is only slightly greater than the thickness of the molding.
Mark all cuts on the back side of the molding. This makes for a cleaner-looking cut because the teeth pass through the underside of a board, showing less wear and tear to the face of the wood.
Splice all pieces of molding with a 45-degree angle cut. Do this by setting the circular saw to an angle of 45 degrees and cutting each piece with a carpenter's square and the saw set at a 45-degree angle. Check that the two 45-degree cuts line up to fit in a complementary way. To make sure the two spliced pieces fit together properly, you can make the first cut with the whole piece of molding placed face down and extended to your left side. Then cut the second piece with the molding placed face down and extended to your right side. For these cuts, you do not have to change the angle of the blade or turn the piece of wood over.
Cut the molding for an inside corner. For an inside corner, set the saw blade to 45 degrees. Set the first piece of molding on the work surface, back side up and extending to the left. Set the blade to 45 degrees and cut the first piece from the back side with the molding extended to your left. You will be cutting the right edge of the length of molding. This piece will fit on the left side of the inside corner.
Walk to the other side of the saw horses and again extend the whole piece of molding to your left. This piece of molding is actually extended in the opposite direction, because you have walked to the other side of the horses) and make the cut near the end of the board using a carpenter's square as a guide. The important part is to walk around to the other side of the two saw horses so in essence you will be pushing the saw in the opposite direction. However, the angle of the blade does not change. This piece of molding will go on the right side of the inside corner with the 45-degree angled cut fitting right into the corner.
Cut the corner pieces for a miter joint on an outside corner. This step is almost identical to Step 6, except you must turn the board face up to get the cut you want. That is the only difference. Of course with the board turned face up, you will have to use extra care when making the cut, so as not to tear the edge of the board.