Achievement Unlocker
The Steam Achievement Unlocker feature will automatically unlock your Steam game’s achievements in a way that looks like they were earned naturally.
Configure your settings in Settings > Achievement Unlocker first
You can add multiple games to the achievement unlocker list
Steps
Click in the sidebar
Click and click on individual games to add and remove them
Click and then
Additional Information
You can add as many games to your achievement unlocker list as you like, but to remain looking legitimate, SGI will only unlock achievements for one game at a time. Once all achievements for that game have been unlocked, SGI will move on to the next game in the list.
You can run multiple instances of SGI to unlock achievements for multiple games simultaneously. You will need to pick which games to unlock achievements for in each instance.