I'll describe how it worked for us.
After you order, you'll get an email with your order information. Once you're actually in the parks, you need to show this e-mail (printout or on your phone) to one of the many, many Universal Photos TMs, including the TMs at the ride-exit photo booths. They will give you a Universal Photos card (or more than one, if you request it) plus one free lanyard to hold it (you have to pay $5 for any additional lanyards you want, so it's best to buy some cheap extra lanyards before your trip).
You use these Universal Photos cards basically like magic bands, except photos don't load automatically in the main 2 parks. At many ride exits, you can just scan the card and select the ride photo with your group in it. You should go up to a TM if there is a problem - if you can't find your ride photo, or if your group's photo is obstructed. They can usually find your photo, and they can edit the photo so that you can see everyone in your group.
In our experience, you should NOT try to set up your account before you enter the parks. You really need the physical cards to make everything smooth, and it only takes a few minutes to set up. If you are rope-dropping your first morning and don't want to waste time on this, just make notes of exactly when you rode each ride in the morning, and you can add your Universal Photos deal and all the pictures later in the day. It's quite easy.
Two other notes - (1) with magic bands, we always got all our ride photos with no work (on Mine Train, etc.) Volcano Bay supposedly has the same thing with Tapu Tapus, and we linked our Universal Photos to our Tapu Tapus, but the photos rarely showed up. The ride photos at Volcano Bay are fantastic, so, if you already have a photo package, it is well worth the time and effort -- you just need to remember the exact time you rode the rides, then go to a TM inside Volcano Bay later in the day, and they will help you find your photos and add them directly to your account; (2) the Spider-Man meet and greet in IoA, with a green screen, is really good if you have the photo package.