Once a song is released, anyone can record it. If you want to go the licence route, you would just issue one to each version of the song. You are your own publisher, so you can do it as formal or informal as you would like. Most artists not on major labels (and even some on major labels) sell so few copies, it is usually not worth much except adding to paperwork. It's a personal decision. If you want to fill out paperwork, you could probably google "Licencing agreements" or contact your PRO for this suggestion.

It more or less just depends on how many copies you think they will sell.

MAB