Prepare a water bath canner, jars, and lids before beginning.
Optional: zest the lemons into the sugar, stir, and let sit about an hour for deeper lemon flavor. The zest must be strained back out before canning, so dissolve the infused sugar into the lemon juice and strain the mixture before filling jars. Use organic lemons if using the zest.
Juice the lemons. Strain the juice through a fine mesh sieve to remove all pulp and seeds, then measure.
Combine the strained lemon juice and sugar in equal parts. For ready-made lemonade, add the water now as well.
Warm the mixture gently on the stove, stirring until the sugar dissolves, until it reaches about 190 degrees F. Do not boil.
Strain again if needed so the mixture is a clear liquid with no solids, then ladle the hot (but not boiling) lemonade into prepared jars, leaving 1/4 inch headspace. Seal with two-part lids to fingertip tight.
Process half pints, pints, and quarts for 10 minutes below 1,000 feet in elevation, adjusting for altitude (see notes).
Turn off the heat and leave the jars in the canner 5 minutes, then remove and cool on a towel for 12 to 24 hours.
Check seals. Refrigerate any unsealed jars for immediate use. Store sealed jars in the pantry and refrigerate after opening.