I used those pictures as a guide some years back to modify a SK. I cheated, no rasp, a Japanese saw and a sander. I measured down to where I wanted the shelf and cut into the riser. I will be honest, that first cut was mentally hard. Once I cut out the wood I used my orbital sander to clean up the cuts. I also reshaped the grip to be more user friendly. I then used wet/dry sandpaper and Tru-oil and sanded the areas baby butt smooth.
Lowering the shelf gets the arrow closer to the bow hand. When the bow is canted the arrow stays closer to the hand, making the arrow stay in line with the target.