When I switched to Signal I enforced it hard. Refused to talk to anyone on another platform. If they texted me, I'd responed, but always add "I'd prefer if we discussed this over Signal."

I had more success than I'd anticipated. About 90% of my family and 80% of my friends moved over.

My best piece of advice for this is to start new group chats - people want to be in the group chats, and if they believe Signal is where it's all happening, they'll join.