First, the person prepping you will bring you into the room with a large metal table, it will have at least one strap on it, but maybe two. It looks scarier than it is. The doctor will have you lie down on a table with your feet on the bottom where there is a ledge. They will secure the straps across the front of you. These straps are good for you, they prevent you from falling or hurting yourself if you faint. They will hook you up to heart monitors that should look familiar, but they'll be the ones with sticky tabs on your skin, they don't hurt! They will put an iv in your arm for fluids, but won't start them yet. (I honestly think the iv prick is the worst part.) They'll take your BP and then you're ready to go.
Next, the actual test starts. They will slowly raise the head of the table until you are being held up by your feet on the ledge on the bottom. Remember the straps are there so you can't fall if you faint, so there is no reason to be scared. They will talk to you and monitor you for a set period of time (about 15 minutes). If you faint, they lie the table down and give you iv fluids until you wake. If you don't faint, they may give you nitroglycerine under your tongue by spray or a tablet (both have no flavor). They will have you stand for additional time after the nitro to see if you faint. Again, if you faint, you get fluids and lie down. If not, they lower the table and your test is negative.
My experience was pretty much as described. I started getting dizzy and woozy when standing but didn't faint. They sprayed the nitro in my mouth and I was out in a minute. If you have fainted before, it feels just like that. If you haven't fainted, it can feel strange. It is different for everyone. Some people get physical signs before they faint, like sweating, heart beat changes, and tunnel vision. Some people have no warning. But in the TTT, the fainting isn't nearly as bad because there are fluids in you immediately, you're not going to get hurt, and they lie you flat quickly. I understand being apprehensive about a test you have never had before, but its something that you can do. We're all strong and brave POTSies!