To use a template, go to the template page, click "Edit", and select and copy all the text in the edit window. Don't click Save on the template page. Now Go to edit your new page, and paste the template text into the edit window.
Many templates include <editing instructions formatted like this>. On your new page, Delete the <italic text> (including the two quote marks at each end) and replace it with text appropriate to the instruction. e.g. replace <Profile Editor> with John Doe. Note: there are often editing instructions at the bottom of the page too.
Consider leaving the link to the source template at the bottom of your new page. Then when you look at a page and think of an improvement, you can easily go to the template and improve it too for the next person.
Templates are just normal pages. Any page with [[Category:Template]] added to the bottom will appear in the list below. So don't forget to remove the Category:Template line from the bottom of your new page since it used the template but isn't itself a template.
Consider naming your template something like Brief Proposal Template rather than Template:Brief Proposal. This allows the alphabetized list below to be more useful.