Native Americans (First Nation Peoples) used to prefer the most tannic/bitter acorns to leech as they produced the best results. However any will do. Tannic acid is (in high enough quantities) toxic so the idea is to leech out as much of this as possible. Although some is left – much like tea or coffee – to give it some bite. Shell, dice, grind, mince – then put into a mesh/net bag or a cloth bag and soak or leach (preferably in running water – or you have to change the water regularly) for a week. Then drain and roast until dark brown. You may use a coffee grinder to get the right grind. Then – make some coffee.