Place clean, fresh or frozen cranberries in a medium saucepan and add water and lemon juice.
Place the pan on the stove on medium-high heat and bring the mixture to a boil.
Reduce the heat to medium-low, cover, and simmer for 10 to 15 minutes.
Remove the pan from the stove and use a potato masher or heavy spoon to mash the berries, ensuring all the cranberries have popped open.
Carefully pour the cranberry mixture into a jelly bag and let it sit for several hours or overnight until the juice has stopped dripping. Use several layers of cheesecloth and a strainer if you do not have a jelly bag.
If canning, prepare a water bath canner, jars, and lids before you start working with the extracted juice to make the jelly.
Measure the amount of liquid you have, then measure the appropriate amount of sugar. If you do not have 2 cups of liquid, add bottled juice to obtain 2 cups, and measure 2 ¼ cups of sugar.
If you have more than 2 cups, multiply the amount by 1.125 to determine your needed sugar.
In a medium saucepan or jam pot, stir the cranberry juice and sugar until the sugar is dissolved, and cook on medium-high heat, bringing it to a full boil, stirring often.
Continue to boil the mixture for 10-15 minutes or until it reaches 220 degrees Fahrenheit on a candy thermometer.
Pour the hot jelly into prepared jelly jars using a canning funnel, leaving ¼" headspace, then wipe the rims of the jars with a clean cloth.
Secure lids to the jars and continue the canning process using the hot water bath method. Process jars for 10 minutes, or 15 minutes if above 6,000 feet in elevation.
If you do not plan on canning your jelly, leave the jars to cool on the counter for 24 hours or until the jelly has set.
Jelly that has not been canned should be stored in the refrigerator for up to 1 month or in the freezer for six months. Jelly that has been processed and sealed using a canning method can be stored in a cool, dry place, where it will maintain quality for 18 months. For all jelly, refrigerate once opened.