Be on the winning side in 20 games of Team Fortress 2.
This achievement will generally happen over time if going for the other achievements.
If you are really desperate you can boost the achievement in the following way.
By selecting the system link option from the main TF2 screen you can actually play in a game by yourself.
So select the system link option and host a new game. If you are purely going for this achievement select a capture map like "Well" then capture the 3 points and you have one win. Repeat 20 times.