It's not that hard, a bit of experience with a soldering iron will do.
The capacitor is a foil type 1.0µF B32529 and the diode a silicium type 1N5408 (1kV) or 1N4007 (1kV), or equal, for sale at the local radio shack for less then a €.
I'll try to make a picture of one of my pumps this weekend. T'ill then; a picture I found on the web from a pump whith the diode only mod. In red I marked the position of the capacitor. Note that it is not possible to solder the diode this way on the aftermarket contacts (won't hold the soldering tin). But you can easely solder it together with one end of the capacitor to the little 'flap' at the side (yellow arrow). At the other end of the capacitor I soldered a small ring-terminal together with the - wire, and use the original screw. (For isolation of exposed wires use heat schrinking tube.)
Now short & simple: a diode is a component that let the 'amps' through in one direction only (see diagram below; in the direction of the arrow). It got a mark (silver ring) on the side from where it 'blocks' the amps.
This side have to point to the + (the wire from the fuel relay).
In a diagram it looks like this:
The diode leads the EMF away from the contacts, and on top of that, the capacitor reduce the spark between them. (Now to be honest, I think that a diode only already have a significant effect on the life of the contacts.)
Btw, it'll all fit, but don't force the cover on and be sure that no moving parts touch the components & wiring!