Almost every figurative painter throughout art history has at some point painted fabric or drapery. Drapery can add mood, tension and correct balance in paintings. Oftentimes, students will study drapery before studying the human form. It is helpful to have images of painted drapery and fabric as references while you work.
Observe the fabric you plan to paint for several minutes. Make a note of each individual shape you see in the fabric. Look at each value of the individual shapes in the fabric. Find the highlights, shadows and mid-tones.
Draw a light sketch in pencil or charcoal of the fabric onto your canvas. Draw the outlines of each shape only. Do not shade your shapes.
An under-painting will help designate your value scales and can be utilized as a map. It is easier to correct an under-painting than a variety of colors. In this case, the under-painting is made of raw sienna, which will help to set the charcoal or pencil into the canvas.
Make a value scale--which shows a range of colors from light to dark--of paint on your palette. Start with white, and add in tiny amounts of raw sienna, mixing the colors together with a palette knife until you reach the mid-tone of raw sienna out of the tube. Each value you create should be a small blob of paint on its own. After you reach the mid-tone, begin mixing small amounts of black paint into the raw sienna until you reach solid black. (Black paint is mixed using ultramarine blue and burnt umber.)
Looking at the actual fabric as a guide, assign a value color that you have mixed to each shape you see in the fabric. Usually a mid-tone color is the majority of the fabric, with lighter values as highlights and darker values as shadows. Keep a keen eye on reflective shadows. Usually, there is a black shadow at the edge of where the fabric meets a table, with a thin line of a light color right next to it, going back into a dark shape.
For your under-painting, paints should be thinned with turpentine to make the paint transparent.
Use one of the flat brushes to paint on the under-painting. Use the other as a dry brush to blend the edges of colors together.
When your under-painting is completely dry, mix your color of choice into a value scale from white to black. Thin it either with turpentine or with linseed oil. Glaze on the paint in thin transparent layers to achieve the desired effect.
If this is your first time painting fabric folds, avoid shiny fabrics such as satin.
Move the lighting around your fabric to create different effects. Find a high contrast so it is easy to see highlights and shadows.
Don't be afraid to go very dark and use black. Contrast is what creates interesting paintings.
If your painting gets too wet, leave it and come back. It can be very frustrating to re-work in wet oil paints.
Make sure you are using values from all parts of the value scale, or your painting will look flat.