Coming soon. The first public version is being tested now. These steps will work as soon as the download is live.
Your phone may say Ponder is from an unknown developer. That's expected while Ponder isn't in the Play Store.
Android 8 or newer, and about 1.5 GB of free space. Ponder works best on recent phones; on older ones, writing down a long message takes longer.
Download the new version here and install it the same way. Your messages and notes stay. Never uninstall Ponder to update it; that removes your studies.