Start on the project page that you're interested in applying for.
Step 1: Click "Apply To Project"
Step 2: Follow the steps to apply
Each step and it's status is listed on the left hand side of the page. You'll need to work through each of the steps before you'll be able to submit your proposal. The steps can vary depending on what's required for the project, but you'll know that when everything on the left is checked, you're all done!
Step 3: Click "Submit Proposal"
This will submit your proposal. Clicking this will send it to the buyer. A proposal needs to be submitted to be considered completed.
Whoops I messed up! Can I change my proposal?
Yes you can. If you click into your proposal and scroll to the bottom of the page, you can click "Unsubmit Proposal" up until the proposal submission deadline.