You will need to know where the stringers and any other braces, fuel tank etc are under your floor. Mark out their positions in chalk on the floor. You don't want to contact anything when you drill a test hole. Sometimes you can use a stud finder but that does not always work.
Once you have figured out a location where there are no obstructions beneath the, you need to drill a test hole. Cut the hole as close to to the centre line of the floor as you can. Use about an 1 1/2” hole saw and once you get the saw cutting, remove the drill bit and then cut the hole. Ease up just before going through the floor. Check for obstacles around the hole with a gas welding rod or even a straightened out metal coat hanger (if you can find one). Mark the location on the floor, with chalk, any obstacles you encounter. You should now be able to determine the maximum size hole you can put in the floor.
You can now determine what size waterproof deck hatch you need to get to fit. Have the deck hatch and a suitable sealant and mounting screws at hand before cutting the hole.
You now need to mount the bilge pump(s) and float switch(s) in the bottom of the hull. There are a couple (probably more) ways of doing this but in my opinion mounting them on a fairly heavy, (the larger the better) stainless steel plate allows for the easiest maintenance, and will keep them in place. Wire up the pumps, switches etc. according to manufactures instructions, being sure to waterproof each connection and not allow the wires to lay in the bilge.
The outlet hose(s) should run to the side of the boat, not the stern, to prevent a following sea from entering the bilge through the discharge hose.