The citric acid solution is a more potent activator than the lemon juice, so basically it is like taking more Miracle-Mineral-Supplement . Not sure exactly how much more powerful, but it is something to be aware of. You will notice that the activated Miracle-Mineral-Supplement is a darker yellow, and smells more pungent.
As far as mixing the solution, just do 1 part citric acid to 9 parts water. You can use tablespoons, teaspoons, whatever.