In a stainless steel pan, combine sugar and water. Bring to a boil, stirring until sugar is dissolved. Reduce to low heat and keep warm until needed, but don't cook the syrup down. Allow 1 to 1 1/2 cups syrup for each quart.
Immerse the peaches in a pot of boiling water until the skins start to split—30 to 60 seconds. Immediately dip them in cold water. Slip the skins off, cut in half, and remove pit.
Optional: To prevent browning, submerge the cut fruit in a mixture of 1/4 cup lemon juice and 4 cups water.
Wash the jars and lids in hot, soapy water. It’s fine to reuse jars, but discard jars with cracks, chips or other nicks, as those will crack in the canner.
Heat the jars by simmering them in water that fills them at least 2/3 full. Keep them in warm water until packing time. No need to sterilize the screw bands.
Immerse the lids in hot but not boiling water. This also helps loosen the adhesive that’ll form the seal. Boiling them can damage the adhesive's ability to seal properly.
Fill jar with peaches, face down and cover in syrup to the appropriate recommended headspace. This headspace amount matters. The recommended headspace for quarts is 1/2 inch.
Air bubbles are a big factor considering the cavity that peach halves create when placed face down in the jar. Use a clean, sterilized plastic utensil to slide between the interior of the jar and fruit.
Add more syrup to meet the recommended headspace before wiping the rim and placing the lid on with magnetic utensil. Screw on lid, not too tight, just fingertip-tight. Some venting will need to happen while in the canner.
Place jars in canner, completely covered in water. Bring to a boil and process quart jars for 30 min. Only start timing after it comes to a boil. Remove canner lid and let them sit in the water for five more minutes. This is part of the processing time. Remove jars, leaving the pool of water on top (resist the urge to tip the jar), place on a towel with some space between jars. Let sit undisturbed, away from any drafts for 24 hours or until all the lids have sealed. You’ll hear them pop as the vacuum inside sucks the lid inward. Properly sealed jars have a concave lid that’ll pop when you open them in the middle of winter.
Eat with homemade whipped cream within one year of canning.