Hi DP: We just started to customize our layouts for our users (we are a recipient.) My comments may not help if you are publishing.
Recipient or Publisher, you need admin permission. Do you have that? If not, then you need to talk to the admin.
First question I ask my people when they want a custom attribute is why? Is it an existing GDSN attribute? Will the supplier publish to it? If it is a custom attribute that will only reside in the DSD, its fairly easy to set up. Many of the common things are already built in the tempaltes within the layout section, so you can clone and not repeat. Keep in mind, some attributes can become obsolete so if you are cloning a less common attribute, it may be turned off at some point in time.
If you are still thinking add an attribute, then, start at page 105 of the Admin guide (I am referencing 8.7 v.2) and begin reading. Watch your validation rules and good luck.
My hindsight lesson; check the existing attributes first by searching basic XML names in the search function on the Attributes and Layouts section. We created about 40 "custom" attributes only to realize they all existed and we had to delete them and reconnect using the proper GDSN attribute. It was a great lesson on how to set up your systems. Luckily, we have a very patient onboarding team at DSD!
