Any advice on removing the cap-like plastic nut that holds the fuel pump in the tank?

Tools to use, techniques? Warnings, etc.

 

Normally there is a special tool you need.You may be able to make one your self.It looks like a half of a square and has a slot on each side to lock into the ring.You may be able to tap it loose with a screwdriver and a rubber mallet.

I would remove the fuel lines first.I use a set of long 90 degrees needle nose pliers.Lift up the outer sleeve on the feul line to release it then lift up on the line to remove.

Pay attnetion to the position of it as well and make sure the new one matches the position as well.