First, VERIFY YOUR PROPOSAL with the "Check Proposal" button. This process verifies the completeness of key fields (e.g. an Abstract is present and Observations defined). Also, generate a summary using the summary page and check it visually for errors.
You should take as much care over proof-reading your proposal as you would have had were it being submitted as hard-copy.The tool will check for example that an Abstract has been completed, but it will not check it for spelling errors, or scientific validity!
The verification process may generate warnings or fatal errors. Warnings are an indication that you may have omitted a field (e.g a target list) but in some cases this is acceptable (in this case, if your proposal covered more than 30 objects). A propsal with fatal verification errors cannot be submitted. A proposal with verification warnings will generate an "are you sure" dialog before it can be submitted.
Once you are sure of the completeness and correctness of your proposal, submit it by hitting the "Submit Proposal" button. After this the process may take a minute or so. Once complete, a dialogue box will be displayed indicating the success of your submission. If successful, you will be allocated a reference number to identify your proposal to the GBT.