By opening signal, putting in the username and sending a text. Signal only uses MMS as a fallback when communicating with someone not on signal. When both parties are on signal SMS/MMS is not used. Presumably they are OK with not being able to communicate with people not on signal.

Why do you need to text? Sending messages without the arcane UX and unreliability of old school SMS or the tries at improving it(RCS) is simply better.