Below, the standing line is marked with an X and should be going to the tip of the rod that would be  in a rod holder.  The bent rod would keep pressure on the standing line like a third hand.

With my method you need two props, a vertical rod holder and a place to sit close to it. Your legs and body need to bend at 90 degrees as it would normally in a chair.
Put your rod in the rod holder and pull of as much line as you will need for a double leader. Lock the reel and double the line.
 

#2
Put the loop over your knees and slowly spread your knees apart to hold the loop.  With your right hand grab the tag end and pull slowly at 90 degrees while spreading your knees. This will compress the twenty twists.

#3
You will now feel tension on the tag end, let the tension pull line back slowly over the twist until it reaches the Y of the loop.  Pinch the twist with your left hand and with your
right hand, make a half hitch around one side of the loop line, then the other side.  Now make three half hitches around both lines of the loop, reversing it once.

#4 & 5
Note: With the rod tip, keep tension on the standing line to the loop at all times.  Having a free hand is what makes the knot a lot easier to tie!