Oil the body. Pour some oil in your cupped hand, and then rub your hands together, this warms the oil up, and helps you to be able to spread the oil evenly and smoothly over the body. Don’t squirt the oil directly on the body, this can be very disconcerting for your partner, and therefore counter productive to your aim of giving a relaxing massage.
Start off gently and smoothly. Warm the muscles, like you warm up before you start exercises, and give yourself an opportunity to feel the body and understand it. Note any hot spots or tense muscles.
As you warm the body this is a good opportunity for you to be aware of your thoughts. Aim to feel calm and relaxed yourself, as this all assists with the relaxing experience. I like to imagine that my movements are like waves over the body. Long gentle rhythmic strokes soothing away the stress and worries of life.
Ideally do a basic massage course to give yourself a few tips and massage techniques.