Install Simple Demo from the Android Play Store.
While making a new account in the app, enter a 10-digit phone number. If you don't have an Indian number (starting in +91) just use a fake 10-digit number.
Create any 4-digit PIN code.
When choosing facilities, pick any of the "Demo" facilities (e.g.: Demo Facility A, Demo Facility B etc.)
Ask someone from the Contributors team to give your Play Store email address access to the app. Unless this happens, you will not be able to find the app on the Store.
Once given access, you should be able to join the Sandbox app access program.
After you've joined, you should be able to see the app on the Play Store.
While making a new account in the app, provide a correct phone number: you will receive a verification OTP SMS on it.
When choosing facilities, pick any facility. It doesn't really matter.
Tell someone from the Contributors team to approve your account. Unless you get approved, no data will sync to your phone.
Read this Google Doc for testing instructions for specific features of the Simple Dashboard