This is an abbreviated how-to–if you want to see the complete how-to including variations and untying instructions and more, check out my knot course, here.
How To Tie the Prusik Knot Video Instructions
How To Tie the Prusik Knot Picture Instructions
Step 1: Use a loop of rope (sometimes called a sling) tied in a strong bend (such as the double fisherman’s knot). and place it with the knot side down over the rope you want to tie the Prusik to (I’ll call it Rope A in this example).
Step 2: Take the knot of the sling and take it behind Rope A and bring it over
Step 3: Take the knot of the sling and wrap it in the same direction around Rope A while keeping the top of the sling in the same place with the other hand.
Step 4: Wrap the sling one more time around Rope A
Step 5: While keeping the ropes all arranged parallel without letting the ropes cross, Begin to pull the bottom of the sling. The top of the sling should come down in a kind of smile formation.
Step 6: Continue to pull the knot end of the sling to tighten while keeping all the lines parallel. I admit that this knot is a bit fiddley, and you have to mess around with it to get it right.
Step 7: Continue pulling the knot end of the sling until all the slack is gone from the top of the sling.