To treat chafing it depends where it is on the body.
Generally for the armpits, breasts and inner thighs once you have washed the skin with a non-residual soap or gel, thoroughly dry the area and then apply a cornflour mixture (as outlined below) using either a powder puff or a shaker.
Mix together these essential oils and apply to clean skin:
10 drops lemon
20 drops lavender
20 drops grapefruit
10 drops juniper berry
10 drops patchouli
From the internal perspective, ensure your diet contains essential fatty acids or fish oils, and little or no processed food and sugars, and that it is high in fresh food, salads and protein.
Keep your skin hydrated by drinking 6-8 glasses of filtered water a day and avoid dairy-based foods and wheat due to their sugar content.