If you purchased Salty Shrimp GH+, that is a premixed mineraliser. That means the manufacturer has premixed Calcium sulphate and Magnesium sulphate together at a ratio of around 4:1. If you want more control over the Ca:Mg ratio or if you want to make GH/KH+ you can purchase these ingredients and mix your own.
I have a sticky post in Water Parameters, if you are ever curious of making your own remineraliser.
If you want a bit more control over the parameters you get from Salty Shrimp ... mix it into a plastic water bottle. Fill 1/4 or 1/3 of the bottle with SS GH+ and add RO water. Dissolve the SS GH+ and drip only as much as you need from the bottle for each water change.