Head to your profile page, and directly below your bio paragraph - the first section will be titled "Tarvaa Gem Archiving".

The Tarvaa Gems held within the wallet(s) connected to your ALPHi account will show here. In the top right of each Gem's descriptive box, there is a button/slider, you can "enable" to archive the gem.

Pay the gas fee and the "soft staking" will immediately begin. Soft staking meaning the gem will not leave your wallet, but will still accumulate PHi constantly.