To give your stained glass design a bit of dimension, you need to try a technique called matting. Some use a stippled matte almost exclusively in their glass painting to prevent a heavy-handed look in the end result. It is also a good deal easier to achieve than a flat matte, especially for the novice.
You may use black or bistre brown for matting. For the sake of practice, mix a batch of bistre brown paint on a new palette (always use a separate palette for each color of paint) and follow the steps used for mixing black. On another clean piece of glass and using your small brush, quickly apply bistre brown paint over the entire surface. Then, while the paint is wet, take a large soft blender brush and gently sweep vertically over the glass, then horizontally, blending the paint as evenly as possible.
This sweeping motion should be made from the elbow to give an even blending of the paint. If done from the wrist, the paint will be moved in localised areas. This can be used to give a fine graduation in tone, but is not suitable for an even blend of paint over an area.
When the paint begins to dry in streaks, immediately begin a gentle up-and-down pouncing motion on the glass. This stippling technique creates a pin-hole effect over the glass surface allowing light to sparkle through. It helps avoid the muddy opaqueness resulting from simple blending that often destroys the translucent beauty of the glass; and the effect of good glass painting.
If you like the "look" of matting and intend to use it often, you should acquire one other brush that will serve you well - a 3"-4" badger hair blender. Nothing accomplishes even blending as well and as quickly. Blenders are, however, a bit pricey. These brushes are an investment well worth the cost and they will last for years with good care.
It would be helpful for you at this point to fire your practice pieces in a kiln to check the results of your paint mixing and painting techniques. The black and brown pigments are high-fire paints and will need to be fired to 650°C (1200 °F) or a little higher for float glass. Correctly fired colours should acquire a translucency and should clearly appear to have fused with the glass.