When you want to create a new proposal, click on the 'New Proposal' button in the top right corner on the proposal list page:
Clicking the button will send you to your saved templates list and ask you to pick one to use. You can also click on the Gallery to view and import more templates.
If the proposal you're creating is very similar to a past one you wrote, you can choose to duplicate a past template.
Pick a template you want to use and click Edit to begin editing the template. You can click Generate Proposal while editing to convert the template into a proposal.
After this point you'll go to the proposal setting screen to enter a few details before delving into the editor so you can start writing your proposal.
You can also choose to start one from scratch, in which case you'll just go to the setting screen, and then when you edit the proposal you can create new sections or import sections from the content library.