Help – Process for selling On Mercari

Hey so I just sold something on Mercari for the first time. Payment hasnt come in yet so I won be shipping it immediately rn but can someone help me and explain to me how im supposed to ship it. I have no idea or I have a slight idea but its not enough. How do I ship it through the combini?
Please help

UPDATE:

I've shipped it! Went to combini and scanned the QR code, got the receipt and gave it to the cashier who then helped me wrap the label around and then took it. Thank you for the advice and info you guys gave, it helped me know what I was doing 🙂 Also, I used a regular envelope since i used らくらくメルカリ便.