In the left panel open the Templates tab and click Save Current as Template. In the Save as Template pop-up enter a Template Name (for example Wedding Proposal v1), a Category (default Custom; categories group templates in the Templates tab) and a Type (Full Proposal or Section / Block Group), then click Save Template. The message Template saved confirms it and the Templates list refreshes. This works both in the standalone Proposals View Builder and inside a proposal's Visual Proposal Builder -- both save into the same shared template library, though the per-proposal builder posts to its own save endpoint on the proposal.