Remove the petals from the dandelion heads. You can do this by patiently pulling out the petals, or you can use scissors to cut the petals off at the base of the flower. You want to be left with only yellow petals.
Take about 2 cups of your dandelions (reserving a small handful for mixing into the final jelly) and put into heatproof bowl or jug. Pour boiling water over the petals until they are just covered. Allow to steep until it reaches room temperature.
Strain the soaking liquid through a fine sieve to remove the petals. Measure out the liquid - you should have about 1.5 cups. If you have less, add a bit of water to make 1.5 cups. If you have more, don’t sweat it. It will simmer away in the next phase!
Combine the liquid, lemon juice, pectin and sugar in a large saucepan.
Boil until jam reaches setting point (I use the wrinkle test, see notes).
Take the pot off the heat. Stir in the reserved petals.
Pour into hot jars. Leave the lids off for now. Check the jars after about 20-30 minutes. If the petals have risen to the top, you can use a small spoon to stir the jam in the jars to redistribute the petals. It’s no big deal if it sets with the petals on top, it just looks a lot nicer when they’re distributed throughout the whole jar!