There are two teams.
Team 1 has the front yard and Team 2 has the back yard, or a field was split between the two teams. The teams are given a time period, like 5 minutes, to hide their flag in their part of the yard.
[optional] During this period spies were sent out to see were the flag was hidden as well as look-outs to catch the spies.
When the flag is hidden you call out that you are finished. Then you simply try to get the other teams flag. If you get caught and tagged by the opponent on their territory you had to go to jail and could only be freed by a teammate who grabs you when your opponent isn’t looking.
The first team to capture the flag wins. In most versions you had to both get the flag, and bring it back to your side.