Use a framing square to make perpendicular lines on lumber by butting the lipped edge of this tool against the side of the wood board. Then, using a pencil, draw the line using the other leg of the framing square as your guide.
A framing square looks like a large right-angle triangle with two equal sides. When the lipped edge butts against the side of the board, you can use the hypotenuse of this triangle to draw lines that are at a 45-degree angle. On the hypotenuse of the metal triangle, there are also marks that delineate a full 90 degrees. The corner directly opposite the hypotenuse is the pivot point.
To use a framing square to draw different angles, butt the speed square against the lumber with the lipped edge along the top of the board. Then, rotate the triangle along the pivot point in an upwards direction until the top edge of the board reaches the desired degree mark on the framing square's hypotenuse. Then draw a line from the pivot point along the leg of the triangle, not the hypotenuse, that is still against the board to make the angle.
To use a framing square as a cutting guide for a circular saw, place the framing square against the piece of lumber with the lipped edge firmly against the edge of the board. Then, while still holding the framing square in place, butt the saw shoe against the edge of the framing square. Make the cut with the circular saw using your other hand.