A retainer example
Here's an example of how retainer billing might work for you.

Let's say you're a graphic designer and have a retainer agreement with Acme Widgets to provide 20 hours of time each month at an hourly rate of $100.00. The first thing you would do is create a billing category to record retainer amounts. Each month you would add an entry to your "Acme Widgets:Ongoing work" project/matter, using your "Retainer" billing category, and an amount of $2,000.00.

Then for all Acme Widgets work, you add entries using a "Design-Retainer" billing category which applies to the retainer by default. (A setting you made when you created or edited the category.) The only charge on the project for time is the $2,000 retainer fee.

Since your retainer agreement does not cover expenses, any expense entries for Acme Widgets would be added to the $2,000 retainer total each month.

Since your retainer agreement allows you to charge for time beyond the 20 hours, any work after you've reached $2,000 for the month is entered with a different billing category that is not applied to the retainer (or you can change individual entries' retainer status), so that they too will be added to the $2.000 total for the month.

If your agreement with Acme Widgets provides for a separate retainer amount for individual projects, you can create separate projects for each retainer.