According to support on thetileapp.com, it's fairly simple:
Adding a Tile to your app is easy! Just follow these quick instructions for each new Tile that you would like to use:
- Add a Tile: Tap the + button to add a Tile.
- Press the "Tile" button or "e" button firmly on the Tile until you hear it play a tune.
- Place the Tile on your device as directed by the app.
- Pick what your Tile is going to be attached to
- Activate your Tile.
- Be happy because now you can always find your stuff!
Unfortunately, this doesn't seem very definitive or precise, so I did a bit more research, and found this article on howtogeek.com, which breaks the activation process down a bit more:
- Download and install the Tile app
- When you open the app, log in, creating an account at need
- Make sure Bluetooth and GPS are enabled
- Tap the + symbol at the top of your screen
- Confirm your e-mail address if you haven't already
- Select the model of Tile you have and add it to your account
- Click the button on your Tile device in the center under the logo
Text in full:
To set up your Tile, you’ll need to download and install the Tile app for Android or iOS. When you first open the app, you’ll need to create an account, if you don’t already have one, and log in.
You’ll need to make sure that Tile can access your phone’s Bluetooth and your GPS location. If you’ve turned off either, the next page will ask you to turn them on. You’ll also need to keep in mind that Bluetooth and GPS are necessary to locate and track your Tiles. If you ever need to turn them off to save battery, you won’t be able to ring your Tiles.
Your phone will be added as the first Tile device on your account. Even without a Tile Mate or Slim, you can use the app on the web to find your phone. Click “Got it.”
Tap the plus symbol at the top of the screen.
If you haven’t confirmed your email address, you’ll need to do so before you can add any Tiles. You’ll receive a six-digit code to the email address on your account. Enter it to continue.
Next, choose the model of Tile you have to add it to your account.
For the next step, grab your Tile and click the button in the center underneath the logo. Put your Tile next to your phone to pair it. The app will also demonstrate on your screen.
Hope some or all of this helps!