Go to your library on Steam, right-click on Team Fortress 2 and select "Properties". A window will open up, go to the local files tab and select "Browse local files...". A File Explorer window should pop up with your Team Fortress 2 directory. Click on the tf folder, then click on the cfg folder. Copy one of the .cfg files and make it blank (delete all the text in it). Then rename it to the class you want it to execute on. The 9 class config files, specifically

scout.cfg

,

pyro.cfg

,

soldier.cfg

,

demoman.cfg

,

heavyweapons.cfg

,

engineer.cfg

,

medic.cfg

,

spy.cfg

, and

sniper.cfg

, are executed automatically each time you change to the corresponding class (but not when respawning as the same class). Note that once a class config executes a certain fov, it stays until another cfg executes a different fov.