Working carefully, separate the elderflower blossoms from their stems, making sure to remove all green parts.
Pour 4 cups of boiling water over the flower blossoms in a heatproof container and let the tea infuse for about 10 minutes (I like to use a large Pyrex measuring cup with a pouring spout for this step).
Carefully strain the elderflower tea infusion into a saucepan using a fine mesh strainer.
Stir the lemon juice into the elderflower infusion and bring the mixture to a boil.
Add the pectin and stir to dissolve.
Let the mixture boil for a full minute, and then stir in the sugar. If the pectin and sugar are added at the same time the jelly won't set, so make sure these pectin and sugar are added a full minute apart.
Bring the mixture back to a boil and cook for 1 minute.
Ladle the elderflower mixture into 1/2-pint jars, leaving 1/4-inch headspace at the top.
If canning the elderflower jelly: apply 2-part lids until fingertip tight and then load the jars into a hot water bath canner; process for 10 minutes.
If you don't want to can the elderflower jelly, simply apply 2-part lids and let the jelly sit at room temperature until cool. Store in the fridge for up to a month or in the freezer for up to 6 months.