The setup and requirements for base capture is the same as the setup for capture the flag, you need a field, two flags and some players. Flags can be as elaborate as a 3x5 foot flag on a pole or a simple as a plastic bag hanging on a tree. You should have at least four players but you can play with as many as you want - just make sure that your field is large enough to handle your numbers and not too big for a small group.
Establish two bases at opposing ends of your field and position a flag at each of these bases (hanging from a tree, on a flagpole, etc). Divide your group into two teams and position each team at a base. Make sure that both teams know where both flags are positioned and that the field boundaries are clearly defined. Before you begin, also establish a time limit for the game - twenty minutes usually is about right, but depending on the number of players it could be longer or shorter.
All general rules of paintball apply, and remember to be safe. When both teams are ready, the game begins when someone shouts, "Game on!" The goal of each team is to capture the opposing flag by simply touching it without getting hit. A player is out and must leave the field if a paintball breaks on him or he calls himself out for any reason. A team may not move their own flag under any circumstance.
A team wins by eliminating all the players of the opposing team or by touching the opposing team's flag.
Since there is no need to return the flag to your own base, base capture sometimes requires you to gather your courage and charge like mad for the flag. The secret to victory is not to carefully plan a prolonged assault on a base but to simply plan a quick attack that will allow one player to get close enough to tough the flag.