A field and some players. You should have at least three players per team but you can play with more. More than about eight to ten players per team might be too many.
Establish two bases at opposing ends of your field and position. Divide your group into two teams and position each team at a base and have each team elect a VIP. Make sure that the teams know who the VIP is on the opposing team 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 eliminate the opposing team's VIP while protecting their own. A player is out and must leave the field if a paintball breaks on him or he calls himself out for any reason. Depending on how you want to play the VIP can play with a gun, but it is often more exciting for the VIP to be unarmed.
A team wins by eliminating the opposing team's VIP.
Trust nobody. Hunt for your target but treat every opposing player as if they have your name.