Hello,
Your stealthtext Android app encrypts your transaction and sends it as a text message to a number you specify. This number must be a Google Voice number, that simply transforms your text message to an E-mail. Other providers can be implemented as well, as we all saw on the Iranian guy's video (Google Voice is not available in Iran). Then you have to setup some sort of listener on your mailbox, when a text message arrives the transaction is made. This is done by a Thunderbird plugin and StealthRelay. You have to configure Thunderbird to fire up StealthRelay when a message with a specific filter arrives. StealthRelay can decrypt the transaction with the info you provide in it's config. Then StealthRelay communicates with your wallet using RPC to do the transaction intended.
This pipeline is not so hard to setup. A tutorial was made available when stealthtext was released.