Rub pork all over with seasoning and set it in a slow cooker, fat side up.
Add water to cover pork about halfway.
Cook on high for 4 hours, or on low for 8 hours.
Remove pork and shred, discarding large fatty bits.
While pork is cooling, set corn husks to soak by setting them in large pan and pouring hot water over the top. Set a heavy dish on top to completely submerge.
To make the masa dough, combine masa, baking powder, salt, and chile powder.
Add water and mix to make a moist dough.
In a large bowl, beat the vegetable shortening until light and fluffy.
Add the masa mixture to the shortening and beat until shortening is fully incorporated and dough has a spongy texture.
To assemble the tamales, select a corn husk, shake off excess water, and lay flat. Spread on a layer of masa vertically from the middle of the husk to the wide edge.
Add pork in the middle of the masa, then roll up from one side and fold over the skinny end. Set into a steamer rack, folded end down, and repeat until masa is gone.
Steam tamales for 60 minutes, or until the masa releases easily from the corn husk when unrolled.
Notes
You will have enough pork to make approximately 5 dozen tamales; make more masa as needed or save the pork for another use.