How should I crumble the substrate?

There are several ways, all of which are similar. Birth the cake, and simply pull it apart with CLEAN hands. PF cakes feel soft and gummy/Styrofoam -ish, so you really will need to *pull* them apart if they are adequately colonized. The more a cake is colonized, the more effort is required to pull it apart (up to a point). You can also birth the cake into a large ziploc bag, and pull it apart through the plastic - which eliminates the amount of handling required (and therefore reduces the possibility of contamination as well). When casing, you will need to remove any primordia (pinheads, tiny mushrooms) that have started to grow on the cake. If these are left on, they will rot inside the crumbled casing, creating a new vector for contamination within the substrate layer of the casing.