Step 3: Tie Prussiks
These knots called prussiks (shown below) self-tighten and grip onto a rope when you step into the loop, but can easily be loosened to slide around.
You tie the prussik by slipping a loop of rope through itself a few times.
If you know what a lark's head knot is, tie that, but slip the rope through the loop more than once to get this.
For this, three turns were enough. Four was disastrously frictionful, too much to be useful for climbing this tree.
I started with four and reduced it to three.
You can read more about prussiks here.