Prepare the jars and canner. If canning, fill the water bath canner with water and place it on the stove to bring it to a boil. Then, place the rings and lids in a small saucepan filled with water and bring them to a simmer. Keep the clean jars warm in the oven set to 200 degrees Fahrenheit or hot water.
Make the jam. Combine the halved strawberries and lemon juice in a large saucepan, mashing the berries slightly. Gradually sprinkle the pectin over the berries, stirring well after each tablespoon to prevent clumping. Bring the mixture to a full boil over high heat. Once boiling, stir in the granulated sugar. Return to a hard boil and maintain the boil for 1 minute, stirring constantly to make sure the mixture does not scorch.
Assemble jars. Remove the jam from the heat. Skim off any foam with a spoon. Then, ladle the jam into the prepared jars, leaving about 1/4 inch of headspace at the top. If any jam remains that does not fill a jar, transfer it to a smaller jar or container and refrigerate to use first. Wipe the rims of the jars with a damp cloth to remove any residue. Securely place the lids and screw on the bands until they are fingertip tight.
Process the jam. Carefully lower the filled jars into the canner's boiling water, making sure they are completely submerged. Then, cover the canner and process the jars for 10 minutes. After processing, remove the jars using jar lifters and place them on a clean tea towel on the counter to cool. Leave the jars undisturbed for 24 hours. Check seals; the lid should not flex when the center is pressed.
Store or Serve. Store sealed jars in a cool, dark place for up to one year. Any jars that do not seal properly should be refrigerated and used first.