Aaaw, the bimini twist. It can be a very decptive knot to tie; however, it is not as difficult at it initially appears to be once the fundamentals are rehearsed.
After the 20 twists and holding the tag end at a 90 degree angle, you must expand the loop and then let the 90 degree angle line rotate around the main line until it is complete at the bottom of the loop. It does take practice to insure the loops are tight against each other. The expansion of the loop in a deliberate fashion is the key to insuring the loops off the 90 degree line are correct.
Once you have the bimini twist completed, it is a good idea to put a lock in it. This is a three turn uni-knot at the end.
In case you don't have Lefty Kreh/Mark Sosin's book on knots, I suggest it in the strongest way.