They are made by adding garlic to a restore potion (which is made adding harralander and red spider's eggs to a vial of water). This gives 25 Herblore experience. Silver dust should be added to the unfinished potion to make a Guthix balance, also giving 25 experience.
Garlic and silver dust can be added to restore potion of any dosage; i.e., 1-4 doses. The dosage of the unfinished potion, and finished Guthix balance, will be dependant on the original dosage of the restore potion. The number of doses for these unfinished potions is not shown in their names, although it will be for the finished potion. The experience for adding the ingredients is the same, independent of the dosage.
For maximum efficiency, it is recommended that restore potions are decanted into 4-dose potions before Guthix balance potions are created. Alternatively, if being made from scratch for the experience, making the potions at the default 3 doses would earn more experience at the cost of more materials.