To create a report, open your project and click "Reports" in the left nav bar.
In the Reports area, click +Add Report.
You will then see all of your Report Groups populated in a list.
Select the Report Group that you'd like to use, re-name the report as needed, then click Generate.
This will render a PDF preview of your report.
Reports are static, so any changes you make to your project will not affect the reports you have already rendered. You can always render new reports after making changes in the Builder.
Hope this helps - let us know if you have any questions!
Here is a tutorial video on creating proposals in the Designer: Proposal Designer