Ask in plain words
Say what people will receive, roughly how often, and how to stop. Put that wording right beside the box they tick, and leave the box unticked so the choice is really theirs.
Give a reason to join
People sign up for something useful: reminders, early notice of offers, or answers to questions they actually have. Tell them which of those they will get.
Keep the record
Store when each person signed up, where, and the exact words they agreed to. If a customer or a carrier ever asks, you can show it in a minute.
Send what you said you would
If the sign-up said a few messages a month, keep to that number. Sending more than people agreed to is the fastest way to lose subscribers and invite complaints.
Make leaving easy
Honour STOP and unsubscribe straight away, everywhere. A clean list of willing readers is worth more than a long one, and it keeps your messages out of spam folders.