Be specific
"Tell me what you think" gets vague answers. "Add two expenses and split them unevenly on your phone" gets testers doing exactly the thing you're unsure about.
Good examples
- Sign up on a phone and tell me where you got stuck.
- Try adding a task with a due date in the past. What happens?
- Is the price on the checkout page clear before you pay?
After each version
When you post a new version, update what to check so testers focus on what changed. Testers who reported bugs are told about the new version and can confirm your fixes.