I never thought much about water bottle rockets until I viewed a YouTube video on their use in Shields Lab’s virtual summer camp. https://www.youtube.com/channel/UC5Zqx5QCe5b41CcZeWL003g
The camp uses Adafruit’s Circuit Playground Express for numerous coding projects and on day 5, the “launch” the CPEx as a flight recorder on a bottle rocket.
Since many schools will not allow the use of low powered sports rockets, water bottle rockets provide a easy, fun and safe introduction to rocket science.
I recommend using fins from http://Stratofins.com. They make a nice launcher system with pressure gauge that enables more controlled experimentation. But if you are just starting out, you can use Shields Lab’s technique as shown in the day 5 videos.