While logged into the application navigate to System Setup | Clause Setup | Select or Add a Custom Clause Database | Select or add a Custom Clause and Edit.

Once opened, enter [FILLIN#1#Insert Text here] in under the "Clause Text" and replace "Inset Text here" with the instruction text for this fill-in.   

If you want more than one Fill-in in a clause, you would edit the second one to be the next sequence [FILLIN#2#Insert Text here].

Return to the previous screen, and select the "Fill Ins" on the left panel select or search for a Clause and select Edit.  

Within the Fill In Detail Page you are able to add Default text, the application fields, or a Line Item Table.