Grind the pork shoulder in a meat grinder, then set aside.
Finely mince the ham with the garlic in a food processor.
Add the pork shoulder, ham/garlic, and Tender Quick into a large bowl and mix by hand until fully combined.
Pack the meat mixture into a bread pan and cover with foil.
Place the bread pan into a large baking dish filled 3/4 full with water.
Place the baking dish in the oven and bake for three hours. Make sure that the internal temperature of the Spam is 155 F (69 C).
Remove from the oven, uncover the bread pan, and drain the fat into a jar or can to dispose of later.
Cover the bread pan loosely with foil and place a heavy weight on top of the foil. (I put bricks in another bread pan and put that on top of the foiled Spam. You may need to get creative, but I'm sure you can find something in your house that will work.)
Allow to cool, then place the weighed down Spam into the refrigerator and keep it there overnight.
In the morning, your Spam will be ready to use however you choose to use it.
Notes
If you can't find Morton Tender Quick where you are, look for a fast curing salt mix that contains salt, sugar, sodium nitrite, and sodium nitrate.Homemade Spam is not canned, so it will not last forever. Keep it as long as you would keep ham.