You can create custom fields in My Account Settings > Custom Fields. Once created, these will be available as Merge tags for your documents set up in Doc Creator.
After setting up your Custom Fields, you can reorder these as needed based on priority. Simply drag and drop them into the preferred order using this icon.
Then, when you are in your case, you will see these in Case Details. Click Add Form Fields and then select the form that you'd like to create. You can select as many forms as needed.
You will then see the fields populate in the same order as they appeared in Account Settings. You will also notice that only the fields included in the selected document will appear.