Depicting a glass in a painting is good practice in shifting your brain function from the left, logical brain into your right, artistic side. Glasses are often clear, and so they are influenced by the colors in, around and behind them. Reflections and distortions occur because of this; keep your subject simple as you begin your first paintings. Any painting medium can be used to experiment with this technique. Choose colors that are closest to the subject you'll be painting.
Find a glass that has a distinctive shape. Choose a colored glass, or a clear one that contains some liquid. Keep in mind that square or rectangular objects are easier to depict, because they have more delineated edges than round objects.
Draw the outline of the glass object you want to paint, using a pencil.
Simplify the colors you see reflected in the glass. The shapes within the glass will follow the general shape of the object; a long, tall glass will produce areas of long, tall color changes. The same is true of other shapes.
Paint the first few strokes darker than the background. Give color to the area of the glass that is thickest.
Proceed to add colors where you see them, going from dark to light. Keep your painting as simple as possible. Do not belabor one spot, but keep moving and painting what you see. Do not think in terms of an object, but rather in terms of shapes and colors; represent these as faithfully as you can.
Use complementary colors juxtaposed next to each other, when possible. These are colors that fall opposite each other on a color wheel.
Add the lightest highlights last. If they appear fuzzy to you, then paint them as you view them.