The easiest way is to put them in a hot fryingpan untill they puff up and 'pop'. Shelling will be easy.
I have prepared yopo snuff with lime and think i did a 3:1 mix of peregrina/lime, using just 5 beans:
The shelled bean was powdered and mixed with lime, and enough water was added to make a thick paste. This was left for a while, returned to a fryingpan and heated to dry, and scraped off and re-powdered.
The resultant powder was made into lines and insufflated, resulting in amazing oev/cev's. Comparable to my recent Aya visions, and perhaps in some cases even more beautiful.
I did not use the colubrina variety, as these are not truly "yopo", according to what i read, they are 'vilca,'and unlike yopo, don't contain dmt, or as much?