Pin your hair back against your head if you have long hair or bangs. This exposes more area of your face to cover and prevents hair from smudging your work. Wash your face gently with a mild soap, using cool (not cold) water to temporarily inhibit the skin's production of oil that will prevent the paint from properly adhering to your face.
Blot a soft towel over your face once you've washed it to remove all moisture. Apply a small dab of cold cream to all areas of your face. Use an amount that's roughly half the size of a pea. Rub the cream into your skin but not all the way; leaving a small "film" over your skin will help with makeup application. You'll know you have the right amount when you touch your skin and it feels slightly tacky.
Smear on the greasepaint with your fingers to cover the entirety of your face. Spread it evenly over your skin. Add a bit more to your temples and hair line -- two places where people produce sweat the most. After your face is covered to your satisfaction, blot some baby powder on your fingertips and gently dab your face to add a protective layer that will help seal your skin against sweat.