To use Telegram Applets, you must first add the IFTTT bot to your Telegram account. Here's a video to walk you through the steps of setting this up:
After the bot is added to your account, you will need to make it an administrator for specific Telegram channels that you want it to be active in. Here's a video for how to make the IFTTT bot an admin of a specific Telegram channel or group:
Once the bot is an administrator for a channel, you will be able to select that specific Telegram channel in Applets that you turn on.
If you're curious about how you can get the most out of Telegram Applets, you can visit this blog post to see if any of the options explained there would be useful to you.