Sit two logs parallel on flat ground, so they do not roll, and place the log to be cut across the two. Make sure this log is level on the other two.
Mark the top log with the shape of the lower log using a protractor compass and pencil or crayon. The mark should be in the location you want to cut the joint, usually about 12 to 18 inches from the end of the log, depending on the length of the log. The top log should have a half-circle shape drawn on it to mark where the cut will be.
Roll the top log over and cut a "V" groove with a chain saw, using the draw half-circle as a guide. Make the cut on the inside of the marking, as it is always better to cut too little than too much.
Remove the remaining wood from the cut using a wood chisel or gouge and a mallet. Try to make the cut as smooth and round as possible.
Turn the log over and sit the joint onto the bottom log. If it does not fit, lift it up about one foot off of the lower log and allow the joint to drop down onto the bottom log. This will form a depression into the cut showing any other wood which should be removed. Repeat Step 4 and 5 as needed to finish the joint.