You must have either Atmosphere with sigpatches set up on your switch, ReiNX, or SX OS set up on your Nintendo Switch in order to use Tinfoil. You can grab sigpatches from here for Atmosphere. (fusee.zip)
If you have followed my Atmosphere + Sigpatches guide, you're good to go at installing Tinfoil.
Make sure you delete your wifi entries before installing tinfoil, or have airplane mode enabled. If you did prodinfo blanking from the atmopshere guide don't worry about this.
Step 2. Extract the contents of tinfoil.latest.zip to the root of your Switch SD Card.
Step 3. If you have a tinfoil forwarder on your home screen, delete it. If not, ignore this step (Tinfoil's forwarder would be an app on your switch homescreen. Delete it like you're uninstalling a game.)
Step 4. Open the album and launch Tinfoil Installer, wait for tinfoil to update it then re-launch from the home screen.
Step 5. Tinfoil should be open now. Add shops you want, etc.
I would strongly suggest having either exopshere prodinfo blanking or incognito set up on your switch before connecting to the internet with Tinfoil installed. Use my Tinfoil incognito guide here, or you can use atmosphere's prodinfo blanking info here.