You need to find the appropriate fuel line fittings to make your own homemade gauge manifold, unless you bought the special seadoo factory service gauge tool. A good FLAP (favorite local auto parts) shop should have the type fittings you need to make your own gauge manifold, and the special tool for pulling the special springlock high pressure fittings apart.
Basically, you will insert a "T" in the fuel line that contains your pressure gauge.
From there, I suggest you can follow the shop manual service procedure, you should have a shop manual anyway, as they're worth their weight in gold.
Also you should measure compressor air pressure, a good place to begin.