The quickest and safest method for cutting precise log tenons is with a tenon-cutting jig on a table saw. You can build an accurate tenon cutter with some scrap materials. The cutter is designed to ride along your table saw's fence, guiding a workpiece clamped to the jig through the saw blade and removing the material on the face of the board to form the tenon.
Measure the height and width of your table saw's fence with a tape measure.
Cut a 2-by-6 board into four lengths of 13 inches, and another length of 12 inches.
Adjust the table saw's fence, and convert three of the 13-inch boards to the width of the fence you measured in step 1. Rip the fourth 13-inch board to a width equaling the height of the saw fence, plus 2 1/4 inches. Finally, rip the 12-inch board to a width of three inches.
Align the three matching 13-inch boards so that they are flush on each edge, and glue and screw them together into a block.
Position the widest 13-inch board along one side of the block (covering up the three boards), flush with the top and edges. The lower portion of this board will hang down below the block a distance equaling the height of the saw fence. Attach to the block using glue and screws.
Place the block assembly onto the fence, with the block riding on top of the fence and the wide board hugging the right side of the fence. Position the piece of plywood against the left side of the block and fence, aligning the front and back edges. The bottom edge of the plywood should rest on the saw table. Attach the plywood to the block with glue and screws.
Push the assembly forward and back along the fence, checking to see that it glides smoothly with no impingement or excessive play. Make any necessary adjustments before the glue dries.
Stand the 3-by-12-inch board vertically, and align it flat against the face of the plywood, flush with the edge closest to the front of the saw. Make a pencil mark on the face of this board, four inches up from the table. Attach the board to the plywood with glue, followed by screws placed only above the four-inch line. This board will serve as a stop to help hold the tenon board in position.
Place a board with the tenons marked vertically onto the face of the plywood, aligned with the stop board. Clamp this workpiece to the jig.
Adjust the position of the fence so that the right side of the saw blade is aligned with the left tenon mark on the board. Clamp the fence in place, adjust the blade height, and turn on the saw.
Ease the entire jig forward to make the cheek cut on the face of the tenon. Slide the jig back to the starting position and repeat with the cheek cut on the opposite side of the board.