It really depends on the software, control panel, and host you use. I am no expert with setting up webmails and stuff, but I do know that you can install a widget depending on the website software you use or you can write your own one in. This widget has to connect to your control panel and create itself an email and fill in the password given. Then your website has to have a place where they can log in and read the emails. Theres no one, two, three steps that I can give you. Sorry, I hope that I helped.