Need tutorial on how to hook up a fuel pressure gauge to 2010 Prius

That diagram just comes from the section on fuel pump "on-vehicle inspection" in the repair manual. You'd want to look over that whole section for the context.

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You'll see under "2. CHECK FUEL PRESSURE" the first step is "discharge fuel system pressure" and after that you get to "disconnect the fuel hose from the fuel main tube", and both of those steps have blue [INFO] boxes, which you can click to be taken straight to the other sections in the manual that explain in detail how to do them. So that's where you'll see where that hose connection is located, and also the right way to undo it—it's tricky.

At least, you can click them when you're looking at the manual online. If you're looking at a PDF that was made with a bunch of print-to-file from the online manual, then the blue [INFO] boxes are still there but don't do anything. You can usually find the right section by a search on the words preceding the [INFO] box.

Once you disconnect the fuel hose from the main tube, that's where you would MacGyver in a tee so you can both reconnect the hose and have a pressure gauge connected.