First, buy a bottle like the one in this link. Dissolve a 50 mg Naltrexone tablet in 200 ml of distilled water, and keep it in the fridge.
Next, do the math: 4 ml of water equals 1 mg of Naltrexone. Since 4 ml is just under 1 US teaspoon, taking 5 mg of Naltrexone per day would be roughly 4 US...