The very simplest way is to pick up a second bottle of exactly the same
juice, but at 12 mg strength. Dump the two one ounce bottles into a two ounce empty bottle, shake, and you will have two ounces of 18 mg strength.
Next would be get a 0 nic doubler of exactly the same flavor and PG/VG ratio. Use 10 ml of the 0 nic doubler and 30 ml of the 24 mg
juice, shake, and you'll have 40 ml of 18 mg juice.
Beyond the above two choices, if you can accept diluting the flavoring you could add PG and VG directly, but that's normally more of a hassle than it's worth. 6.5 ml of PG, 3.5 ml of VG, plus the original 30 ml of 24 mg juice will give you 40 ml of 18 mg strength, preserving the 35% VG, though cutting the flavor (since you're adding flavorless PG and VG).
Hope this helps.