Sterilize jars: Submerge glass canning jars upright in large pot with at least 1-2" of water of headroom. Bring to a boil for at least 10 minutes. They can stay longer than 10 minutes, just make sure to keep them fully submerged.
Sterilize lids: In a separate pot, submerge the flat part of the lids (not the rings) for at least 10 minutes, or until ready to use. Do not boil!
Carefully remove a jar from the water. Dry out the inside with a paper towel, then pour jam into the jar, leaving at least ¼" headspace at the top.
Run a knife or small offset spatula around the jar to knock out any air bubbles trapped inside. Wipe the rim of the jar with a damp paper towel to remove any debris or jammy bits.
Place one of the flat sterilized lids on the jar and screw the ring piece on so that it's "fingertip tight." (You should be able to unscrew it with little effort using just your fingertips.)
When all the jars have been filled, submerge them in a pot of boiling water with at least 1-2" of water above them. Cover with a well-fitting lid to create pressure inside the pot. Maintain a rolling boiling for 10 minutes, then uncover and turn the heat off. Let sit for 5 minutes, then remove the jars to a clean towel.
The lids should be perfectly flat on top or will "pop" flat within a few minutes of being removed from the water. Once they do, tighten the rings the rest of the way. Caution: Jars will be hot! If the lids don't pop flat, try flipping them upside down to cool. Otherwise repeat the canning process, knocking out air bubbles and making sure there's no debris in the way of the rubber seal.