I measure the amount of salt with a kitchen scale. Then I throw the salt into a bucket and add RO water. Stir it up with a plastic spoon a few times, and that's it.
In the last time, I often add a powerhead to the bucket when it's 10 liter for the nano. I never do for this pico as I measure the 2 liter of RO water for that not with the markings in the bucket but with a specially marked 2L-plastic bottle.