1. Prepare — and check the situation first
Wash your hands and file your nails smooth; a rough edge can scratch delicate tissue, and small cuts bleed more readily when arousal increases blood flow to the area. If you have any cut on your hand, use a barrier — a glove or finger cot.
Before anything else, run an honest check: if there's a history of pain with penetration, pelvic-floor discomfort, recent childbirth, or a possible infection, read up on pain-first approaches or talk to a clinician before trying internal touch at all. This framework is for situations without those flags.
Then check the simpler question: is tonight a good night for both of you? If either of you is unsure, that's your answer — this keeps.