At this stage insert the marble into the center.
At this step you have few options to end the monkey fist, you can make a simple knot, snake knot, diamond knot or any other finishing method, you can find guide for Diamond knot and more at my blog:
Congratulations on your first monkey fist.
Steps For Diamond Knot:
1. Create the diamond knot (see guide image).
2. Before tightening the knot slide in the 2 end cords. (see marked image).
3. Cut the 2 end cords close to the knot.
4. Lighter, water to dip your finger in (when fusing), pointy object to push warm cord into the knot.
Start by fusing together the 2 end cords, try doing it neat and carefully to get nice finishing. When both ends are fused warm them with the lighter and push them in the knot. Now you can attach a ring to it (or before) and pull the ring and the bottom 2 cords to get the final tightening.