you "can" use RO/DI water, but not necessary unless you have unusually hard tap water.
If you want to use it, just mix it with tap water to a point where get to 3 dKH and 7 dGH (obviously, both may not be attainable, but you can correct it as necessary afterwards). Unless one of those goes below 3 or 7 in the meantime, you will not need to add anything else. Baking soda will increase KH, and Calcium/Magnesium will increase GH.
Post what your current tap water KH and GH are and we can help you meet that proportion of RO/tap.
450g Planted Discus Tank - Work in Process!