Hi Dyr. I think I'm on the same journey as you, & skywalkers tutorials have been fantastic (thankyou :-)).
You don't say why you don't want to use the Extended Registration module, also are you still trying for the multi-page? (I gave up on that for the time being, didn't have much success with groupid).
My registration form's still pretty new but I've got a solution that works for me. I wanted to use as much existing orchard functionality as possible so I didn't create my own controller (rightly or wrongly I'm not sure yet), I'm also learning MVC
at the same time as Orchard so it's been very steep.
I ended up inheriting Orchard.Users.Controllers.AccountController with my own controller so I wouldn't have to duplicate the fairly extensive private methods in there. I have 2 actions in my controller, RegisterMember & [HttpPost]RegisterMember,
which both make use of the base Register actions, and then add in the MemberPart bits.
So this all works great with pretty minimal code, but then the next problem is what to do about all the extra non-essential fields I'm adding to MemberPart which I don't need/want to show on the registration form? What I've ended up doing is creating
url-alternate based views in my theme (not module) to basically comment them out. It took me a while to get my head around this, and as I said it's still pretty new, but I think it works. The end result is that I've got a fairly nice looking one page
registration form that asks for email address & essential MemberPart details.
I now have one more requirement - our members are required to agree to make a minimum no. of offers as part of their membership, so I need to also include some NoticeParts on the rego form, the notices aren't part of the User, only owned by the user, so
that's the next step for me.
Good luck with your form, and if you'd like to see my code, I'll post it here, I only didn't do it this morning cos I'm working on that next step at the mo :-)