How do I send old passport when renewing through signed for service??

So I've applied to renew my passport online but I need to send back the one I have before I can get the new one. Previously I did it through the post office this is the first time doing it online. I want to use the signed for service at royal mail, how would I do that? Do I just put the passport in a letter and put the passport office address on with stamp and then go to a royal mail delivery office and tell them I want to use the signed for service for the letter then pay and leave it with them? Do I need to add a return address too?

Did my mums for couple weeks ago and she’s received it. No stamp needed or rtn address as you would of put this in your application online. Just make sure when you pay for the signed service @ the post office you get a receipt with a tracking number and you’ll be covered!

Thanks, so I just go to the post office with the letter and pay for it there with cash/card or do I have to buy the postage online then go in with proof I've brought postage?