You will need to remove the stock venturi/nozzle and turning mechanism from your jetski. After that you will be able to mount the jet pump adapter plate to the back of your jet ski with the included bolts.
SeaDoo: If you have a Sea-Doo with a 3 bolt pump pattern you will need a special spacer before you are able to mount the jet pump plate. You should have received this spacer plate with your order. Attaching the a 4 bolt Sea Doo pump is straight forward without any extra spacers. Simply place the plate against the exposed jet pump and match the holes of your jet pump with the holes on the plate. Use the included bolts that came with the plate.
Yamaha: You will need to adjust the ride plate by lowering it 1/4in (6mm) in order for the U pipe to have clearance to connect to the jet plate (pic #2). Loosen all 4 bolts for the ride plate (about half way). Insert the two plastic horseshoe shims around the 2 rear bolts. Tighten all 4 ride plate bolts. There is also a spacer ring that must be used between the Pump Adapter Plate and the jet ski pump (pic #4).
Kawasaki: Mounting the jet pump plate on a Kawasaki is the same as on a Sea Doo accept on a Kawasaki F15. Kawasaki F15 uses the same spacer as the Yamaha except the mounting bolts are the same size as the Sea Doo.
Pic 4 (Yamaha pump spacer)