Simon gets LOTS of questions about drum tuning, so here is a tutorial video to show you how to do it!
Over time the skin will naturally stretch a little, bringing the tone down. Djembes need to be tuned periodically (a couple of times per year) to maintain a good sound. Putting knots in the cord pulls the metal ring downwards, increasing the tension on the skin. You can weave as many knots as you like, but be mindful that over-tightening can cause the skin to break.
There are many techniques out there. Simon thinks this one is most simple and effective for tuning djembes by hand. What do you think?