Step 2: Spice
Place 4 cups pumpkin juice into a medium saucepan, add the ginger, cloves, allspice, nutmeg, and cinnamon sticks. Reduce (that is, boil down) the 4 cups pumpkin juice by half. So you should have about 2 cups concentrated, spiced juice.
Remove the cinnamon sticks and strain the juice through a coffee filter that has been placed in a filter holder or mesh strainer. If you are particularly impatient, you can strain it through cloth (cheesecloth is too loose to strain out the spices so I sometimes use clean organic unbleached muslin in these situations). This process can take some time but it's well worth it.
I have previously made this liqueur and aged it with the spice but this made it far too spicy. I find that infusing the spice into the juice during the "reduction" phase gives a more subtle spice flavor without overwhelming the pumpkin-ness.