Scartail
What I did to locate the hinge and bumpers for my seat was start first with the hinge.
Put the seat hinge in place with a strip of double face tape where it will contact the seat base.
Now put a couple of strips of wood (thickness of the bumpers) across the frame to temporarily support the seat.
The strips of wood must sit under the seat base and not interfere with the fit.
Align the seat where you want it and press into the double face tape to position the hinge.
Remove seat being careful not to move the hinge, drill and fasten the hinge.
Next, using double face tape again, tape the two front rubber bumpers to the frame.
Put some masking tape on the underside of the seat, where those bumpers will contact the seat base.
The masking tape should be lightly placed and not firmly pressed on.
Reinstall the seat on the hinge. With the bumpers in place, hinge the seat down on the bumpers and press down firmly.
Remove the seat again and you should see where the bumpers pressed against the masking tape.
Drill the appropriate sized holes and install the bumpers.
Repeat for the two back bumpers.
The lock down pin can be located similarly to the hinge too.
Hope that makes sense and helps.
FJ