In almost all work situations, you need to separate problems and assign priority. Some things are just complete nonsense if you prioritize them wrong. It'll be clear once I call out the two distinct problems you have.
- How to populate the form
- How to meet deadline
In this case, it is nonsense to even try to meet the deadline without doing it right. There's no way that's acceptable, so the deadline is secondary to doing the form correctly.
Regarding how to fill the form correctly, it should be clear how you resolve it.
- Ask someone
And since you're new at this, you'll need someone to review your work and confirm its correctness. You may want to find out who actually uses these forms and whether there are examples of correctly filled ones.
If someone tells you that you have missed deadline, let them know that you do not know how to fill them out correctly yet. Ask them for correct copies of the form so you can use it as reference. Ask them to review your work and help you confirm that it is correct. The goal here is to gain allies while acknowledging your lack of progress.
Whatever you do, do not simply meet the deadline blindly and think that'll solve the problem. You'll definitely make the problem worse, especially if the form is being consumed by a poorly made automated system and corrupts data.
If you are a developer, there will be huge amount of pressure to meet the deadline doing things wrong. I don't need to call out specific examples because the industry already has laughingly huge amount of examples, but it's something you'll have to call out and make sure the person sticking to the deadline knows they're sacrificing quality for speed.