Mary Ellen does such a tremendous job with photos and summaries that I couldn't possibly add much value to the discussion but for those of you TR addicts, like me, who enjoy any/all pictures, I will add mine for your enjoyment.
The Trek does require park admission. You meet with the trek guide just after you've entered through the turnstiles. There are multiple treks offered throughout the day. I think ours was one of the first. I believe "word on the street" is that the earlier treks are 'better' as animals are more likely to be active before the heat of the day sets in and I suppose the same goes for us humans!

It was getting hot while we were out there - especially with those weighted vests on- but not unbearable. I'm not sure if the offered times are extending through the end of the day but I also think sunset would be a wonderful time to try this. The views from the savannah "rest stop" are just spectacular!
So you start out getting outfitted for the trek. The requirement is to wear close-toed shoes. FYI, in case you forget they do have extras for loan. I think it was Chatty Cathy/Know it All Nancy who didn't have the right footwear and they were able to accommodate her. You can only take with you on the trek what you can clip onto your vest. So if your camera has a strap, you can take it but if not (i.e. iphones etc.) you can't. You are given a locker to put your belongings in that you won't be taking with you.
You also get a silver thermos for water - you can see them in some of the pictures - to take with you on the trek and keep as a souvenir.
I forget the order in which things occur but you get a bit of jungle juice to start the trek off and also practice crossing a practice bridge so that you know what it will be like and I suppose test that you can make it through without freaking out. The planks are not evenly spaced so you need to pay attention but not so far apart that you are jumping. And there is net underneath the planks such that even if you missed the step you still wouldn't be plummeting to your death or engaging the harness.
Before we departed into Harambe we stood in a circle and introduced ourselves, saying our name and the state we were from. The guide remarked how we were from all over but still friends and how did we meet? None of us wanted to admit to being "crazy internet friends" at that moment.
Walking through the park was funny. You could definitely feel people looking at you and wondering what in the heck you were doing. We also got earphones/radio devices so that we could always hear the guide no matter where we were in the group. That was very helpful.
You start by trekking through the woods and come across some interesting finds. Quite a coincidence to find a skull here, don't you think?

Believe this is a hippo skull in keeping with the spot we were about to visit.
Soon enough we were visiting the hippos. Here we had to attach our harnesses to some contraption and that allowed us to lean over the edge of the "cliff" to get a better view of the hippos. I still had two feet firmly planted on the ground but could lean forward to the point that you felt the harness was holding you, if that makes sense. The hippos appeared to be conditioned to their daily visitors and opened wide as soon as they saw watermelon. Hard to believe these cuties could crush you to death, but that is what I'm told.
I just love their little whiskers!
This shot give you a better perspective of how close we were without the zoom:
One last shot of the cuties. They were adorable. It was funny because one of our cues to leave was one of the hippos relieving himself... apparently there are some things Disney can't control!
More to come...