How do I pay full or partial retention on a Sub Contract?

You can go to the Bills module and Create a New Bill.

Select the Project, Subcontractor and Bill number (all the mandatory fields with the red asterisk). Once done, click on the button “Add item to Bill” and at the bottom of the options shown in there you can click on “Add Retainage (Partial or Full) for payment”.

A pop-up window will be shown. Type in there the retention amount and click on “Save & Close”.

A line item named “Retainage Payment” will be added to that bill with the Retention amount we typed on the previous step.

To complete the whole cycle, you can post a payment to that Bill (once you have actually paid your Subcontractor) by clicking on the “Action Items” (the Three Dot icon next to the Green Pencil) and click where it says “Post Payment”.

Also, see how to Create a Sub Contract  and read the article Bills (Overview)