Step1: First you need two pieces of timber and on the first piece you mark two lines which are going to be spaced out as far as the thickness of your second piece of timber.

Step2: The next thing you do is repeat step 1 on your second piece of timber so that the lines you mark out are as wide as the first piece of timber. Once all lines are marked out you will notice that the timber should interlock once the joint has been cut.

Step3: Cut along all of the lines with either a tenon saw or a table saw although a circular saw can be used. Make sure to only cut half the thickness of the timber because if you cut deeper or not deep enough your two pieces of timber will not sit flush.

Step4: Once you have made your cuts you now remove the excess timber with a hammer and chisel which should leave you with a tight and flush interlocking joint.