Painting a glass bottle or jar with oil paint can be difficult for beginners in oil painting. It often requires some level of experience in painting glass on canvas. The most important part in glass painting is to clearly understand the areas that are to be highlighted. To get the natural feel of a glass object, focus on the points of reflection as well as shadows. You may easily find oil painting glass tutorials over web. These step by step glass painting on canvas would help you understand the process in depth.
There are three major values that you should keep in mind. The highlight part of the glass, the mid-tone values as well as the shadow regions. It is often better to work with a colored glass to understand these values in detail.
First achieve the 3D look of a glass on canvas
Try to add reflections (as required)
When you work with a clear glass painting, it is often better to look the object with partly closing the eyes.
Often recommended to learn painting of glass by using objects from life than depending on a photograph.