To create an estimate, just click the "patient context menu" (the dotted button in the top right corner of the Patient Dashboard):
Select that top option: "Build/Manage Estimates", then click the "+" button in the next window, and you'll see this:
Fill in the "Description" line at the top, then you can add an Assessment and attach a test to it, just as you would during the exam. Add any other charges that you want, and click "Create/Update Estimate" at the bottom. Once you've waited a few seconds for QuickBooks to catch up, you'll see this link appear just above that button:
Click the link to go to QuickBooks to view the estimate, which you can then send to your client.