Never submit right on deadline
You should never send a design proposal right on the deadline. It is better, to have a couple of days or even a week between the deadline and your submission as a buffer period.
Sometimes you realize last minute that you are missing important information, maybe your registration documents are out of date or you miss an essential signature. If you still have a couple of days to go, you can fix all these things. If the deadline is looming over you already, you have no chance. You also must contemplate technical problems. If you must submit to a server, it might be down at the time of submission due to too much traffic.
Proofread it
Proofreading is a critical step during the process of polishing your proposal. Do not do it alone though, get people to proofread for you, as you will not be able to see mistakes in your document. Although typos and small errors will not change the content of your proposal, they make up the first impression. If your documents are full of mistakes, the client might think you are careless and will be careless during (the) implementation of the project as well. Be careful to not fall into this trap.
Check all the guidelines again
Make sure you check all the guidelines that are needed and recheck, You do not only have to find typos and mismatches while proofreading, but you also have to check if your proposal is structured according to the guidelines. This includes content, structure, formatting, layout, design principals, and everything else that is specified in the project instructions.
Check if you have all the legal and necessary Documents.
When submitting a design proposal, many times you also must provide supporting documents. These could be your registration certificates, recommendations, or CVs of colleagues. Make sure you have all these documents in the required format and that they are up to date.