No special precautions to mix. You get distilled or deionized water cheap for car batteries. Avoid bubbles when mixing maybe, as they can take time to buoy, depending on the mixture's viscosity.
Glycol is toxic, diglycol less. But they don't compare with novitchok. Glycol is the old antifreeze, so don't take it at breakfast, and wash your hands.
One special precaution: don't let dogs get close to it. They are known to drink glycol because of the sugar taste.
For the proportions, I's just try to match the viscosity, as observed when pouring the original and the copy.