MAKING THE TAIL:
You have a choice to make before unraveling the tail of your RB. Basically, the more you unravel the slower the RB will go - translation, the less distance it will travel. You'll want to unravel at least a few inches to give your RB a fair "tail" for directional purposes. After that, it's up to you. So, before unraveling, wrap your electrical tape at the point you want to stop unraveling, then unravel-away. (You can alway remove the tape and unravel more, but not the other way round.)
Carefully (please), cut along the line you drew on the tennis ball with a (new blade!) box cutter or really sharp knife (I suggest you wear a leather glove on the hand holding the tennis ball). This is not as easy as one might think, so be CAREFUL! A two-inch cut is just about right for the knot at the end of rope. However, you may need to cut the opening a little bit more if you find yourself using profane language while attempting insertion.
While squeezing the sides of the tennis ball near the edges of your cut, use the other hand to push/manipulate the knot of the rope into the opening. Note: Newer tennis balls are more forgiving than older ones. In my experience, older tennis balls are more susceptible to cracking at the ends of the cut (dried out material). If the opening becomes to big it's more difficult to seal it - you decide if you want to try and save a cracked one or not.
Have Fun Out There!