Bubbles are caused by trapping air during the mixing process. When mixing, try to fold the contents, working the glass particles up from the bottom and incorporating them into the liquid, rather than using a stirring motion. Once blended, allow a little bit of time for any trapped air to work its way to the surface. When you are ready to apply the enamels, invert the bottle, hold over a paper towel and squeeze so you release a little bit of enamels as well as any trapped air. If bubbles are still present after applying the enamels to the surface of your object, be sure to use a fine tipped tool to "pop" the bubbles before the enamel dries.
To have a successful fuse between the enamel and glass, you need to allow the enamels to dry thoroughly then fire the kiln for a full fuse. Suggested firing temperature range is 1,300 - 1,550 degrees Fahrenheit/Cone 014. It is recommended to fire any glass product, including the enamel pens on a kiln shelf, as well as to either place a piece of kiln paper on the shelf or treat the shelf with kiln wash before firing to prevent fusing the glass to the walls of your kiln.
Here is a firing schedule for you to test:
Ramp 1 - 300 degrees Fahrenheit
Ramp 2 - 300 degrees Fahrenheit
Ramp 3 - 600 degrees Fahrenheit
Ramp 4 - 600 degrees Fahrenheit
Ramp 5 - full speed
Ramp 6 - 90 degrees Fahrenheit
Ramp 7 - 120 degrees Fahrenheit
Ramp 8 - 300 degrees Fahrenheit