Once you're happy with your sales proposal, click the "Save and Preview" button in the top right corner of the Proposal Editor.
Scroll through your proposal content to have check everything is correct, this is exactly what your customer will see. It is a good idea to also open the PDF version of your proposal (click on the View as PDF button), look at the page breaks in particular and make sure you start content on a new page if it does not look quite right spread across two pages.
Simply press the Send button in the top right corner of the proposal preview window.
A popup window will appear, type your email message (or leave as the default message).
Now click on the Send button and your proposal will be to sent to your customer.
Congratulations! You have successfully sent your proposal to the client.