I also got my potassium nitrate (the one you linked a while back), but it’s in powdered form with no instructions. How should I go about adding it to my tank?
Get empty bottle. Go to http://theaquatools.com/fertilization-calculator/
Select 'dissolved in solution', enter bottle volume (that's just water), tank volume and a convenient amount to add from the bottle.
Aim for ±10 ppm nitrate level. Then start monitoring NO3, and you'll see when you have to add.