Sort of an adventure in stages.
At the beginning, you'll have a few days to go through the schedule and pick out the things you want to try for. Panels, giveaways, signings, etc. It'll include the option for the size of your party. (Some folks put in larger parties and team up in the hopes of trading extra spots.) Then you prioritize them in order of interest. That's the process that starts on the 20th.
After that process closes, they'll do the main drop, and randomly assign you RSPs based on your requests. (There are daily limits on what you can request and receive.)
In my experience, that first stage is really haphazard. Some people seemingly get most of they want. Some people get basically nothing. (I'm in the latter category - I've never gotten anything that I requested.) There usually seem to be certain panels/presentations that are basically the consolation prizes of the main RSP drop - seemingly stuff that people weren't requesting. (Two years ago, a few of us got RSPs for The Princess Concert in the arena despite not requesting them.)
After that, people will start dropping the RSPs that they don't want. If you can catch them, you might be able to pick them up yourself. If you're accustomed to bashing the app for LLs at the parks, you can kind of do the same thing with the app - just sit and poke and see if anything pops up. I'm that kind of person - two years ago, I got most of what I wanted by picking up RSPs over the course of the convention, especially while waiting in line for panels.
Having said that - for the most part, RSPs aren't completely necessary. In most cases, I could have gotten into the panels I wanted without one - it was more of a nice-to-have. But there are a few things that are reservation-only (giveaways, for example).