How do I mix potions?

Ok, so, I am making a block that should allow you to mix potions. Basically, you have 3 slots and a button. In the first 2 slots you put two potions, and in the third one you place a water bottle, then you press the button, and the two effects of the first two potions are applied to the water bottle.

Problem: I have no clue how to do it. I have 0 ideas on how to make the procedure, I don't have a lot of experience with nbt data, and I don't know how to code.

Does someone have any tips on how to do it?