Glass is not an ideal surface for decorating. It's slick and nonporous, so there aren't any spaces for a substance to grip and stick to the glass. It's not impossible to decorate, though, and many people use paint to make glassware more beautiful. If you use ordinary acrylic paint, not designed specifically for painting glass, you need to protect your work to keep it from scratching or washing off. An acrylic sealer is ideal for this, since acrylic sealers are easy to use and clean up with soap and water.
You'll generally find two or three different sheen levels with sealers. Gloss is the highest sheen, with a lot of shine, matte or flat is a low-sheen finish that reflects less light, and satin is somewhere in the middle. For glassware, gloss is the best choice, since a lower sheen may make the glass look dull and less like glass, or may make it look like plastic. Gloss blends in well and makes it appear as though there is no finish on the glass at all. However, since lower sheen reflects less light, you may find it's easier to see your work if you use a low sheen sealer, so take this into consideration when you choose.
For your sealer to adhere well to the glass, it needs to be as clean as you can possibly get it. You'll probably have put fingerprints on the glass during the painting process, and while this is not harmful, it can make the sealer adhere poorly, and you'll trap your fingerprints on the glass, making the glass look dirty. Wipe the glassware down with some plain rubbing alcohol and a soft cloth. Take care not to disrupt your work, and don't touch the glass further while you clean. Wear gloves if necessary.
You have two basic options for applying acrylic finishes. You can use a spray, which works like spray paint, or you can brush it on. Sprays are quick and easy. However, your results vary depending on the weather conditions, since it's best to do this outside, and you can get poor results if the temperature is very hot or cold, or if it's very humid. Sprays also can create unattractive drips if you spray too heavily. Brush-on application is slower, but you can do it indoors, and you can take particular care when you seal your design, applying the finish more gently over the painted parts. Use a soft, synthetic bristle brush for best results. Don't use sponge brushes; these can create bubbles. It's also possible to spray a spray finish into a cup and brush the liquid that collects onto the glass.
Although a sealer makes decorated glassware more durable and protects your work, it's not a perfect solution. Treat sealed glassware gently. To be on the safe side, don't clean it in a dishwasher, as your finish most likely will not be able to withstand the heat or strongly moving water. Wash your work by hand, using a mild soap, warm water and a soft sponge or cloth.