1. Go to Video or wherever you change your game's settings.
2. Most games here will have things such as "Model quality, texture quality, shadow quality, etc." Lower these as you please (preferably to medium) and then click on Advanced Settings.
3. Set antialiasing mode to None. If None is not available, change it to the lowest setting. If this messes things up, change it back to a low setting.
4. Change filtering to None (if available) or Bilinear.
5. Set wait for vertical sync to disabled. If this messes things up, again, change it back.
6. Disable Motion Blur.
7. Lower the FOV to a nice setting where you can still see well but don't see too much so the game doesn't have to process too much on screen.
8. Turn off Multicore Rendering.
9. Turn off High Dynamic Range or lower it and uncheck "Use bloom effect when available. Again, if this makes things look poor change it back.
10.Turn Water reflections to Simple reflections.
11. Turn off Color Correction. If this messes things up, change it back. I don't have much experience with this but you can probably leave it alone, I got my slow TF2 to run just fine without modifying this setting.