Homemade "milk of magnesia" (suspension of magnesium hydroxide in water)
Milk of magnesia is typically just a suspension of about $\pu{8\%}$ wt/v (i.e. $\pu{8g / 100 mL}$) magnesium hydroxide in water. To get a good suspension you will want to have very fine magnesium hydroxide. Ideally, it should feel like flour when rubbed between you fingers. Regardless, there will be some settling and you will need to shake it thoroughly prior to each usage. No other preparation should be necessary.