Before starting, you ensure your hands and nails are well cleaned. File your nails if necessary, so that there are no snags. By doing this you prevent damages on the face and risk of infection.
Always work using clean materials (sponges, brushes, spatulas, water bowls) and change the water regularly.
Get the subject to put on any clothes beforehand that have to be put on over the head. Protect your models clothing with a hairdressing cape or a towel.