As I don't know how to add pictures here, I'll try to explain. The wedge is a bit wider than the footprint of the fork crown shape where it contacts the spring and has notches for the bolts in order not to shift or slide off. Basicly, the wedge needs to be the same shape as the underside of the crown where it supports the spring, in order not to load the crown or bolts in some dangerous way. The wedge angle depends on how much you need to alter the spring aspect.
Basicly when the bike is on the ground, on its wheels with no load, the rockers shold assume roughly the same angle as the lower fender stay or a bit more acute angle. The wheelaxle nut should be partly or fully visible under the fender stay. Certainly not a shallower angle than that or the suspension will be hash, rough and unresponsive on the road.
But first you need to carefully investigate the reason why you need to change the geometry of the suspension. Very small tweaks of the fork shape or spring connecting links does a big difference down at the rockers!
Is the fork bent? A small head on accident can bend the fork backwards. Small differences are very hard to see with the naked eye. You need measurements in 2 or 3 directions with the fork preferably in a jig or on a table.
There are differences in spring connecting links. Are they the right ones for the fork? If so, do they have the right shape? That too is very hard to see with the naked eye. It is crucial that they both are exactly the same length and square to the spring.
Is the spring tired or bent or it's eye bushing worn in some way? In some cases spring leaves can be hammered back cold, but it comes with a slight risk. New spring packs is available, and in fact a complete fork as well.