Fill your canning pot about half full+ with water and set on your stove, covered. Turn it on high to bring to a boil. If it comes to a boil before your beets are jarred, just turn it slightly down to stay hot, turning back up as it gets closer to the time when you're going to process your jars.
Wash the beets, trimming stems to about 1 inch.
Put them in a large pot and fill with water to cover beets. Bring to a boil and boil for 10 minutes. Remove beets, reserving 2 cups of the cooking liquid, and set beets aside to cool. You can also put them into a sink of cold water to speed up cooling.
Set clean canning jars and rings on a cookie sheet, right side up, making sure to leave space around each jar. Also, don't let any metal rings touch the glass jars. Place into a COLD oven and set oven to 225. Once the oven is heated, set timer for 20 minutes. The jars can be in there longer, but they need to be in there at least 20 minutes at 225 to sterilize. Leave them in the heated oven until you're ready to fill them. Adding hot food to cold jars can cause the jars to crack, so you always want your jars HOT before you add any hot food to them.
Place the lids in a small pot just covered with water and bring to a low simmer, NOT a boil. Leave it to simmer until you're ready to use them.
Once beets are cool enough to handle, peel and slice them. They are VERY red and can stain, so I recommend wearing rubber gloves. If you have a mandoline, I use the 1/8" setting to slice them.
In a large pot, bring 2 CUPS apple cider vinegar, 2 CUPS sugar, 2 CUPS of the reserved cooking liquid, and 2 teaspoons of the mixed pickling spices tied into a spice bag, to a boil.
Add sliced beets into the boiling mixture. Boil until tender, about 10-15 minutes.
Using a slotted spoon, spoon beets into prepared pint jars, trying to distribute the amount of beets evenly between the jars. Using the jar funnel, top the beets with the liquid, leaving 1/4" of headspace. Slide the bubble remover/headspace tool into the side of the jar, gently rocking it towards the center to release any air bubbles. Add additional cooking liquid if necessary to bring it back to 1/4" headspace.
Carefully wipe the top of each jar with a clean towel. Using the magnetic lid lifter, retrieve your lids from the simmering water, one at a time. Set a lid on top of each jar and twist a ring on the jar. Just tighten it with your hand, you don't want to over-tighten it.
Using the jar lifter, lower each jar into the boiling water bath. Once all jars are in the pot, on the rack, they should be covered by at least 1" of water. If the water is too low, add additional boiling water until the water level is right. If it stops boiling after adding your jars, cover, bringing back to a boil. Once it starts to boil, keep covered and process jars for 10 minutes in the water bath.
Use the jar lifter to remove jars, one at a time, being careful not to tip the jars. Set them onto a towel on the counter. As they start to cool, you will hear them start to "pop" as they're sealing. After about 20 minutes you can check for seals by pressing down on each lid. If it doesn't move, it's safely sealed. If not, you need to store it in the fridge. Once cool, you can store sealed jars in a cool, dark place for about a year.
Beets are best if you keep them sealed for at least 2 weeks before you open them to get the full flavor. Store in the fridge once they're opened.