Special equipment: Boiling water canner with a rack, canning tongs, canning funnel, eight 1/2-pint canning jars
Place a wire rack in a rimmed baking sheet or line it with a clean towel. (This is to prevent the jars from directly touching the surface.) Set aside.
Sterilize eight 1/2-pint canning jars and lids. To sterilize jars using a boiling water canner or a large pot, place a rack (or often I will use a clean kitchen towel) on the bottom of the canner. Place the jars right-side-up on the rack and fill the jars and canner with enough water to cover the jars by at least 1 inch. Bring the water to a boil, then boil for 10 minutes (at altitudes less than 1,000 feet elevation; see Cook's Note). Using canning tongs, remove the jars from the canner one at a time, carefully pouring the water from the jars back into the canner. Let the jars air-dry upside-down on the prepared rack or towel and sit undisturbed until you're ready to fill them. The rings and rubber-lined lids must be sterilized, too. Place the new lids in a small saucepan, cover with water and bring to a gentle simmer. Very gently simmer for 10 minutes (taking care not to boil). Turn off the heat and keep the lids in the water until ready to use.
Combine the strawberries, lemon juice and sugar in a heavy 6- to 8-quart non-reactive pot and stir well. Place over high heat and, stirring constantly, bring to a full boil with bubbles over the entire surface. Boil for 1 minute, stirring constantly. Remove from the heat and stir in the liquid pectin.
For each jar, insert a canning funnel and carefully ladle in the jam, allowing at least 1/4 inch of headroom. Clean the rims of the jars with a clean, damp towel and tightly secure the lids.
Place the canning rack in the canner and fill the pot with water; bring to a boil over high heat.
Using tongs, place the jars on the rack in the canner. The water should cover the jars by at least 1 inch. Cover the canner. Return the water to a boil and boil gently for 5 minutes (at altitudes less than 1,000 feet elevation; see Cook's Note). Using tongs, transfer the jars to a towel to cool. If the seal works and fits properly, the metal lid will be slightly concave within 24 hours of processing. Store the unopened jars of jam at room temperature for up to 1 year. Once the jam is opened, store in the refrigerator for up to 1 month.
For both 1/2-pints and pints, add 5 minutes processing time for altitudes between 1,001 to 6,000 feet elevation and 10 minutes processing time for altitudes above 6,000 feet elevation.
High-acid foods such as fruits, pickles, sauerkraut, jams, jellies, marmalades and fruit butters with a pH level of 4.6 or lower can be preserved by boiling water canning (low-acid foods, such as canned meats and fish, require a pressure cooker).
Boiling water canning makes use of a large pot that’s tall enough to fully submerge canning jars by at least an inch of water. The pot is used for both sterilization of jars prior to filling and for boiling the jars once they are filled. You don’t necessarily need to purchase a boiling water bath canner if you don’t already have one. Any large, deep stockpot equipped with a lid and a rack can double as a boiling water canner. Keep in mind: The pot must be large enough to fully surround and immerse the jars in water by 1 to 2 inches and allow for the water to boil rapidly with the lid on.
It is not necessary to sterilize jars beforehand if processing jars in a boiling water canner for 10 minutes of longer. The jars should instead be freshly cleaned and well washed in hot soapy water. Any jars processed less than 10 minutes must be presterilized and the lids and rings placed into simmering, not boiling, water. Rings can be reused, but lids should be new and used only once for boiling water canning.