What do you suggest to increase the clarity and quality of the process?
Adding a little "Help" hover button next to each field would be nice. A lot of people seem to confuse what "Bitcointalk ID" means. There should be a help button that explains exactly what the ID is and where to find it.
Also, the user's Bitcoin Talk ID number should be automatically injected into the footer code for them, so that they don't have to manually replace the XXXX, which also seems to be a problem for many to understand.
Sorry if any of this was already added, I haven't checked the registration process in a while.
Concerning the signature set-up there is a great video on youtube, explaining very nicely, how to select the right signature and how to find the Bitcointalk ID and how it needs to be added in the signature. I just also followed it, to make sure I have updated the signature correctly as my Bitcointalk level just changed. So do not forget to keep your signature updated when you change your Bitcointalk Level!