Adding something to an existing page
Whenever you're editing a Proposal or a Template you add something to the page with the green 'Add to this page' tab on the right.
Once the slide-in menu has loaded, click the 'Content Library' tab highlighted in the image below.
This will bring up a search field which will allow you to search for any of your previously saved content. If you're not sure what you called a full with image for instance, you can just type 'image' and that'll narrow it down for you. Same with 'text', 'video', 'feature block' or 'page.
Once you've found the piece of content you want to add, just click on it and it'll insert it into the page you're currently on.
Adding a page from the Content Library as a new page
Click the 'or add a page from your Library' link under the normal 'Add a blank page' button on the left.
This will open a similar sidebar but this time it will only contain pages that have been saved to the Content Library. It works in exactly the same way. You simply click on the page that you want to add and it will add this as a new page to your Proposal or Template.