Once you've logged into your Bonfire account and located the Project you would like to submit your Intent to Bid on, scroll to the bottom of the page where you'll see a Submission header.
From there, click on the Intent to Bid tab and select whether you intend to bid on the opportunity:
If you submit Yes you'll then be able to start your submission by clicking on the Prepare Your Submission button as shown below:
If you submit No, you must provide a reason you are not interested in bidding.
Regardless of your response, you'll have the option to change your decision up to the Intent to Bid Due date.