Cutting a 45 degree angle may be a cinch on a mitre saw. After all that is what the saw is designed to do but you don't always have the luxury of a mitre saw for every project. To cut pieces of wood that won't fit into a mitre saw or to make those mitre cuts while you are away from your workbench, you can use a circular saw. There are two types of angled cuts that you can do with a circular saw, mitres and bevels. Mitre cuts are where you adjust the direction of the saw to create the angle and bevel cuts are where you adjust the tilt of the saw to create the angle.
Place your speed square against the edge of the material you want to cut. Align the pointed edge of the speed square with the exact place you want to start cutting and keep the bottom edge of the square flat against the side of the material.
Draw a line with your pencil along the angled edge of the speed square. If the material is much longer than the speed square, align your level with the line that you drew and continue the line using the level as a guide.
Clamp the material in place and have the part to be cut extending out over the edge of your table.
Use your circular saw to cut the material along the line that you drew. This cut will be a perfect 45 degree mitre.
Clap your material in place with the part to be cut extending over the side of your table.
Use your level or speed square to draw a straight line on the top of the material where you want to make the cut.
Release the bevel adjustment knob on the side of your circular saw. Each model of saw may be a bit different but most will have you unscrew the knob to release it. Rotate the knob, which will begin at 0 degrees on the adjacent scale, until it lines up with the 45 degree mark. Tighten the knob.
Pull back on the blade guard on the front of the saw. When you are making angle cuts this severe, the guard will get in the way of your cut.
Start the circular saw and cut the material along the line that you drew. Keep the base of the circular saw flat on the material at all times.
You can combine these two methods to create cuts that are both mitred and bevelled.