Cut the legs at a straight angle depending on the length you desire for a square, box-framed picnic table. Measure the length of the legs according to your personal preference and mark them with a pencil and carpenter's square. Hold the circular saw firmly in both hands, pull the trigger and run the blade across the cut mark.
Mark the legs at an angle if you want to have the them angled outwards as with traditional picnic tables. A common angle to use in this situation is 22 1/2 degrees. Cut both ends of the legs off at this angle so that the pieces rest flat on the underside of the table and angle outwards to where they will also sit flat against the floor surface. Mark the pieces accordingly and cut them with a circular saw.
Determine the custom angle for legs that are already installed but need to have the bottoms cut off for the table to rest flat. Use the angle finder to determine the angle of the legs. Place the angle finder on the underside of the table and set the angle according to the slope of the leg. Transfer this measurement to the bottom sides of the legs and cut them based upon the angle from your angle finder tool.