If customers are required to make payments on a certain payment schedule, you can setup these entries on the Payment Schedule page.  Add entries for each payment required (e.g. upon contract signing, upon delivery of materials, upon project completion).

Once you've entered the various payments, you can go back in and change the percentages.  With every change the last item in the list will be updated by SmartBuild.  This is to prevent a math error when adding up the percentages.