One calendar, always
Paper notes, a DM thread, and a calendar app will eventually disagree. Pick one place where every appointment lives — for most iPhone users that's Apple Calendar — and treat anything else as a draft.
Book the true duration
A “2 PM haircut” that really takes 75 minutes with cleanup blocks until 3:15. Enter the full time, or add a buffer, so the next slot you offer is actually free.
Confirm last
The safe order is: check → add to calendar → confirm to the client. Reversing the last two steps is the most common cause of double-booking.