DM or distilled water to be used for making the solution, normally 5% conc is required and solution strength can be as per test performance. initial pH to be taken before solution preparation.

may be use Litmus pape

Is there any specific requirement for 5% or 10% solution?

For baby skin powder, make 10℅ solution and then measure the pH with electrode.

Sorry i could not help

An easy and adequate way is to measure pH in the reconstituted milk in the proportion indicated by the manufacturer.

20 g or the powder will be siloved in 100 ml of distilled or deionized water and be sure that all the powder is dissolved in the water, then measure the pH using any pH meter.