Perhaps the simplest way to make a 45 degree cut in a piece of wood is by using a miter saw, but a circular saw and a speed square can also make precise cuts. Miter saws are especially designed to cut at angles.
A miter saw is a kind of powered circular saw that is fixed to an arm that can be positioned at a variety of angles. By changing the angle of the arm, the blade can cut at angles between 45 and 90 degrees.
When using a miter saw, the saw should start at an elevated position, already spinning at full speed. Bring the saw down through the wood at a constant rate of speed, then bring it to a complete stop before raising it again.
A speed square, a right triangle made out of thick metal with units of measurement marked along its length, can also be used as a straight edge to cut 45 degree angles with a normal circular saw. The speed square is placed flat along the wood, and the saw is brought up against the speed square's hypotenuse to make a precise 45 degree cut.