If you are going for a realistic look in portraiture - ignore everything you know about applying makeup!
Beauty tips do NOT apply to the painting of a portrait.
Blush the cheeks with red, but keep it away from the nose and keep it low on the face - right down to the jaw line.
My apologies to the Mona Lisa for using her face as a color map to show where the red really should go.
Don't put any red on the forehead or the neck - be mindful of color banding as shown so well in George above.
Add your reds directly to the wet paint on your canvas - don't mix it first on your palette first.
The nose and nostrils are warm.
And you can add some red on the upper lids of the eyes and in the inner corners and along the top of the lower lid.
You can add red from the lower jaw line right up to (but not beyond) the eyebrows.
Add lots of red to the ears.
You can add even more red to the shadow side of the face.
When you have blended the red in, the effect should be fairly subtle.
You don't want to add so much red that your face looks sunburned. Also, don't cover large unbroken areas in red or else it will look fake.
You want to leave some areas red-free so there is some variety.
You can do the same thing for the hands and feet that you did for the face. You can use use quite a bit, especially on the soles of the feet, palms of the hands and fingertips.
Red likes any place on the human body where the bone comes near the surface - elbows, knees, knuckles.
Sometimes it is easy to add more red to the face of a child than an adult.
I use a warm and a cool red: Indian Red and/or Alizarin Crimson Permanent.