Last week I had the opportunity to teach conjecturally-medieval handball to eight eager students and get them playing the game. Here are my current thoughts on how to get it going successfully.
First, after a very brief account of the early history of the sport, I introduce a historical tennis ball. I demonstrate that, unlike a modern tennis ball, it doesn't bounce very well at all.
I then explain the rules of the game as being, more or less, the same as in modern tennis or ping-pong, but with a few differences. First, because the ball doesn't bounce much, you are technically allowed to return it after a single bounce as in the modern sports, but you're very unlikely to be able to. Second, of course, we don't have rackets or paddles; the ball is struck with the hands. Next, I remind players that all surfaces in and around the playing area are in play, and rebounding the ball off them is legitimate; in fact, there are no boundaries except for the fact that the ball must go over (not around) the rope. Finally, only the side that is serving can score. (I also require shouting when you serve, and suggest shouting "tennis!" as the probable English preference from which the modern name is derived.)
I don't get into the whole system of laying chases, which survives in a few flavors of modern street handball but fundamentally is rooted in the use of dedicated tennis courts. I consider it a singularly difficult thing to work into a pickup game. In teaching, I mention therefore that the later versions, including court tennis as currently played, such a system exists, and is documentably part of other sixteenth-century versions, but that the rules I currently teach are much more accessible, are historically plausible, and appear in several modern variants.
That's enough to get people playing, though I like to actually briefly explain the scoring system. It's just confusing enough, though, that people find it helpful to learn by hearing me announce the score as we play. I'm using the term "limit" instead of "sign" for Vives' signum, because he briefly mentions it as an alternative and it seems to make a bit more sense to people playing today.
There are some gaps in the rules, like whether players can pass the ball to one another on the same team and whether the ball can be struck with parts of the body other than the hand. Any of these that come up and don't have an answer implied by Vives, I defer to the players, saying there isn't a clear-cut historical answer and that I think it should be settled by house rule. (My own preference is for "no" on both of those, but one of my groups unanimously decided to allow passing when the question arose and had a great time using it.)
One more thing - the word "tennis" appears to be a turn-off for a lot of people who enjoy the active leaping after the ball that gives medieval handball its distinct character. "Handball" appears to be a better choice for marketing. Once people are introduced to medieval handball, everyone I've had a chance to play it with seems to have a good time and successfully refresh their spirit as Vives hopes.