If you're water bath canning, start heating the water in your canner.
Heat your jars by keeping them in the water bath canner, a pot of boiling water, or your dishwasher set to sanitize cycle. (You want them hot when pouring in hot jelly, or they could crack.)
Place the 4 cups of forsythia tea infusion in a heavy stockpot.
Add 2 tbsp lemon juice to the tea.
Measure 3 cups of sugar into a bowl.
Scoop 1/4 cup of that sugar and place it into a new bowl.
Add the pack of pectin to the smaller bowl of sugar (containing 1/4 cup) and stir well.
Add the pectin that's mixed with the 1/4 cup sugar, to the forsythia tea infusion and lemon juice that's already in the pot.
Turn the burner to high heat and stir constantly until the mixture reaches a full rolling boil.
Add the remaining sugar and return it to a full rolling boil, stirring constantly.
Boil for one minute, still stirring constantly. The jelly will boil up significantly at this point.
After one minute, remove the pot from the heat.
Working quickly but carefully, remove the hot jars onto a towel, and ladle or pour the hot jelly mixture into them.
If you plan to water bath can your jelly, leave a 1/4 inch headspace at the top of each jar.
Wipe the rims with a damp paper towel, to remove any sticky residue that might interfere with the seal.
Cover each jar with a lid and screw the ring on firmly.
Optionally: For longer storage, water bath can your jelly.