1. Start with words, not bodies
Put the topic into play before anything physical: as shared fantasy during sex, or as a plain conversation outside it.
Talking about it — voicing the fantasy while doing other things you already enjoy — is a complete endpoint in itself, not a waiting room for something more.
Agree where tonight's stopping point is before you start ('fingers only — that's the whole plan'), and agree that either of you can call a stop at any time, for any reason, no explanation owed.
If you'd like a rehearsal without an audience, the early levels work solo too — your own finger, your own pace — before either of you involves a partner.