The bowline knot. One rope, four steps.

1. Hold the line near the end and twist it once to make a loop.
2. Pass the working end up through the loop.
3. Bring it behind the standing part, then back down through the loop.
4. Hold the loop and the working end, and pull the standing part to tighten.

It makes a fixed loop that won't slip, and it is easy to untie after a load. One of the most useful knots to know around boats.