A lot of people are scared away from working with glass because it is brittle, and tends to break under pressure. If you are working with glass tubing, these characteristics can actually be an advantage. When you put a piece of glass under pressure, it will break at the weakest point. By making a scratch in the side of a piece of glass tubing and then quickly snapping it, you can coax it to break exactly where you want it to.
Put on gloves. If you don't know what you are doing, you can cut yourself while cutting glass.
Get your glass cutting supplies. A small triangular file will let you cut glass tubing to approximately the right size. For more accurate cuts, buy a glass gauge cutter like the one linked to below.
Brace the glass tube against the cutting edge of the tool. If you are using a gauge cutter, simply put the glass tube in the cutter and close it with slight pressure. If you are using a file, pinch the sharp edge of the file against the glass tube with the thumb and forefinger of one hand.
Make a scratch in the glass. Either rotate the glass in the cutter, or against the file to make a deep scratch. You do not have to make a scratch that goes all the way around. Just scratching half the glass should be enough.
Break it. Grab the glass firmly in both hands and snap it with a sharp yank. With some gauge cutters, you can also break the glass by just pressing down on a lever.
Use a quick snap. Slow, building pressure is more likely to shatter the tube or get a jagged, uneven break.