First off, some of the "basics" you hopefully already know. The Wizarding World Harry Potter is split amongst both theme parks. Hogsmead and Hogswarts Castle with Harry Potter and the Forbidden Journey is in Islands of Adventure and Diagon Alley with Gringotts Bank is in Universal Studios Florida. The 2 areas are connected by the Hogwarts Express. So, to see everything you will need Park-to-Park (Park Hopper) tickets. Also, the Journey on the Hogwarts Express is different in each direction.
Right now Early Park Admission is available for Diagon Alley and Despicable Me to onsite guests, individuals a universal package staying at a partner hotel and Annual Passholders. APH Early park admission is valid through Dec 18 with Nov 17 and 24-28 blacked out.
Touringplans.com offers a good Harry Potter + Best of the rest one day Touring plan you might want to look at. It also does personalized plans and the price is pretty cheap especially with a Discount code. In general, if you don't have early entry you start at USF ride a few of the headliners then ride the Hogwarts Express over to Hogsmead. Visit Hogsmead, ride a few of the Headliners at IOA Ride. Ride Hogwarts Express back to USF Hit Diagon Alley Finish the day with Cinematic Spectacular show.