Multisite and domain mapping confusion!

Full diclosure: I'm a complete newbie to multisite and domain mapping and not very tech-y at all!

Aim: To host and build a wordpress site for a friend

I already host my own domain (memmalomas.com) on GoDaddy (it's fairly new).
My friend asked if I could create a site for them. I said I would.
They own a domain but haven't got hosting.

I was hoping that with multisite, I could set up their site from the network dashboard and build it using a theme just like I would with my own site.

I wanted my friend to have their own login details for the site - although they won't use them because they want me to manage it for them.

I also wanted to use a sub domain of my own domain (memmalomas.com) but when visitors go to my friend's site- they would see friendsite.com not friendsite.memmalomas.com

Not sure if this is possible now - but when I was looking into everything I thought it was?

Anyway:
I've installed multisite.
I've installed domain mapping.
I've added a subdomain on Godaddy in the cpanel
I've added the subdomain on domain mapping
I chose the subdomain rather than subdirectories option when setting up domain mapping (I read afterwards that GoDaddy make this a pain in the *ss to use).

Yesterday it was saying that the website didn't exist - today it does exist but I can't access my network admin :slight_frown:

In short - I've clearly made a huge mistake - and I haven't got a clue how to fix it.

Can anyone give me step by step instructions that a 5 year old could follow?
Maybe a checklist to see if everything is in the right place...

Thanks in advance!