When we send a playtest out to our players we give them a short email briefing. It contains the following information:
Which of their devices they should use.
When the playtest starts (especially important for multiplayer sessions).
The steps they must take to complete the playtest properly.
We also send the player a video with information on how to playtest. This includes:
Talking aloud about everything they see.
Not to focus on a specific topic for too long.
To play the game as they would as if they had just downloaded it from the App Store or the Play Store
That the game might be an early prototype.
If you chose to give the players a custom briefing it will come after our default briefing. This is a good place to leave any special instructions you have for the players. You can also use Markdown to add images and formatting to the custom briefing.
If you would like to learn more about player briefings, have a look at this article.