Priority is given to people who have over 100 posts on TSR. You can use the service with fewer posts, but you will have to wait a while - so don't leave it til the last minute.
You need to make a thread in the Submitted PS forum. It's a private forum, so only you and the PS Helpers can see it (there are loads of threads in there already, but you can't see any of them). This means it won't come up on UCAS plagiarism checks. Make sure you write the subject you're applying for in the thread title. Add the unis you're applying for in the main post. Write "this is my final draft" at the end of the post .