Hi Manuela :waving:
You're doing the exactly the same thing I did nearly 2 years ago, I was stuck on the dressing gown and my hubby found this site where I got good advice.
The pattern isn't easy to read. Yes the collar is made separate and out on afterwards.
Here is a link to a thread I started way back, there is lots of good advice on there;
And this is my finished gown;
Good luck! I'm currently working on the short version for someone (even though I vowed NEVER to do touch anything like that again!)
If you need any more advice give me a pm.