Keep it simple. Here's a framework you can use in conversations:
What it is: "It's an authorisation for us to collect payments directly from your bank account on agreed dates."
Why it's safe: "It's protected by the Direct Debit Guarantee. If anything goes wrong, your bank refunds you immediately. You can also cancel at any time."
Why we're using it: "It means you don't have to remember to pay us every month. It happens automatically, and it saves both of us time."
What they need to do: "All you need to do is fill in a short online form. It takes about two minutes. We'll send you the link."
Most customers are already familiar with Direct Debit. They use it for their phone bill, utilities, and subscriptions. Framing it this way makes it feel normal, not new.