Nikostar

Scarcely Lethal Noob
Start a new session in SFM, load the map that you want to use. Then just pose all your characters and/or objects that you want in the way you want. Then go to File, and go down to Export, then select poster, choose file type, size, save location, etc. When you have finished, try something like Picmonkey or Photoshop to add effects if desired. Take a look at my profile if you want examples, or just look in the Team Fortress 2 artwork section on Steam, it's probably better to look there as I am still relatively new to poster-making.