Sterilize the jars and lids - place them in a pot of boiling water for 10 minutes. Also, place a small plate in the freezer (unless you have a candy thermometer to test the setting point of the jam).
Clean, pat dry, and roughly chop the rhubarb into 1-inch slices.
Place the rhubarb in a large saucepan with the sugar. If you're using powdered pectin, add it now (if you're using liquid pectin, you can add it later).
Heat everything up gently over medium-high heat, stirring occasionally, until the sugar has dissolved. You can mash up the fruit, if you like, to your desired consistency. Once it's all mixed together, squeeze in the lemon juice and turn up the heat.
Let the mixture boil for about 10 minutes, scooping off any scum/foam that comes to the surface. The fruit should be nice and soft by this point. If you're using liquid pectin, add it in now and let everything boil for an extra minute.
Test for the setting point of the jam. Get your chilled plate from the freezer and take the jam off the heat and spoon a little bit onto the cold plate.Let it sit for a minute or two, then push your finger through the jam. If it stays put and doesn't flood back in to fill the gap, you're good to go! If not, keep cooking for a couple of minutes at a time, testing in between. Alternatively, if you've got a sugar thermometer, you're looking for a temperature of 220ºF/105ºC.
Once the jam is ready, let it cool down. Then ladle it into your warm, sterilized jars. Leave about 1/4 inch of space at the top of the jar. If you see any air bubbles in the jam, gently tap the jar to get rid of them. If you need to adjust the headspace, just add a bit more jam. Wipe the rim of the jar to make sure it's clean, then place the lid and screw it until it's just finger-tight.
Notes
Once cooled to room temperature, place it in the refrigerator. You can use it within a few weeks. If you water bath can it, it can last for 6 months or longer. Check the blog post for more tips, water bath canning instructions, and serving recommendations.