First off, take a deep breath - we'll get you through this.
DHS is scheduled to open at 8am on Wednesday. I would keep a close watch on this thread for Monday and Tuesday times/operations to help provide a framework for what Wednesday may look like. Thursday - Saturday the park opened for guests at 6:15am, 6:09am, and 6:36am respectively - and Boarding Groups were immediately available to join at that time. While we don't know when the park will open Wednesday, for the moment I would plan on a similar approx 6am opening just to be safe, unless we see something otherwise the next two days.
Once you enter ("tap in") the park with your ticket admission, you then open the app on your phone, click the "Star Wars: Rise of the Resistance - Access by Boarding Group" graphic that should be on the opening page, then click "Join Boarding Group" and follow the subsequent instructions. It may take the app a minute or so once you scan your park ticket admission at the tapstyles for the system to recognize you're in the park / let you join a Boarding Group. Everyone in your party needs to have scanned their ticket admission (eg, be in the park with you) to join a Boarding Group.
When your Boarding Group is later 'called' you should get a push notification to your phone letting you know, or you can easily check the app at any time. You have 2 hours to return to the ride once your boarding group is called.
Ignore whatever is leading you to the "refresh from 7:30am onwards" comment above - that was something unique to today's EMH which won't be applicable to your day.
Josh/EasyWDW had a good post yesterday with alot of the specifics and screen shots of the app/Boarding Group process:
https://www.easywdw.com/easy/blog/d...after-star-wars-rise-of-the-resistance-opens/
A similar step-by-step overview from Blog Mickey:
https://blogmickey.com/rise-of-the-resistance-virtual-queue-boarding-groups-guide/
Hope that helps, if anything isn't clear please ask.