Follow the steps in order for the best result.
In a large, heavy-bottomed saucepan, combine the prepared zucchini with the pineapple juice, lemon juice, and sugar. Stir well to ensure the sugar begins to dissolve.
Bring the mixture to a rolling boil over medium-high heat, stirring occasionally to prevent sticking.
Once boiling, reduce the heat to low and simmer gently for 20 minutes, stirring periodically.
Carefully ladle the hot zucchini mixture and its liquid into sterilized canning jars, leaving a 1/2-inch headspace at the top of each jar.
Remove any air bubbles by gently tapping the jars on a towel-lined surface. Wipe the rims of the jars clean with a damp cloth.
Place sterilized lids on the jars and secure with canning rings, tightening until fingertip-tight.
Process the filled jars in a boiling water bath for 15 minutes. If you live above 1,000 feet in elevation, increase the processing time to 20 minutes. For elevations above 6,000 feet, process for 25 minutes.
After processing, turn off the heat and let the jars sit in the hot water bath for 5 minutes before removing them.
Carefully remove the jars from the water bath and place them on a towel-lined surface to cool completely. As the jars cool, you should hear a popping sound as the lids seal.
Once the jars are completely cooled, check the seals by pressing down on the center of each lid. If the lid doesn't flex or pop, it is properly sealed. Store sealed jars in a cool, dark place.