you need a pH meter. Some pH up and down. I use vinagar and baking soda.

test pH of water
you get: 5.5
put a small amount of pH up or B.S. in the water
mix well, let sit so it can mix.
Text pH of water
you'll get a higher number say 6.0
(i always test twice)
another little pH up or B.s.
Test water pH 6.9 (too high)
add a drop of vinagar

continue untill desired pH is reached. It's really easy. but always try to add as little pH up and down as needed. so try to not over shoot. In a .5 Liters of water i find 1 drop of vingar will change the pH by an entire Point!!!

if you use too much baking soda or sodium bicarbonate. the molecule disassosiates in the solution in the soil and the Na can bond to things like potassium.