After saving/creating a proposal you will be redirected to the preview area where you can add your content. However if you are trying to add content to a specific proposal or came back later after saving a proposal then your have to open the proposal first. Navigate to Sales > Proposals and find the proposal and click on the proposal number. The proposal should appear on the right side pane which would look like the following image. Now simply click on the text area and start typing. As you click on the the editing area the pane will be expanded to full width.
The system will automatically save your content every few seconds interval.
Merge Fields/Short Codes
There are list of short codes available for you to use. Click on the Available Short Codes link and then on the right side click on any of the short code to add it to your content. In case you are not sure what Merge Field/Short Code is then please check out this article : What is a merge field or short code