couple of ways - some use propane torches with wands of glass; and some use extremely hot fire ovens used just for glass blowing. i've seen both ways in action. it is quite the artform and would take awhile for someone to explain it in writing - i'm sure it is best to see it in action from those that know how to do it . Can get burned really easily too!
The only way to blow glass effectively is by using a furness. It is a serious art and is highly skilled and dangerous. There are 2 methods used dependant on what is being made. What Relache is referring to is the easiest in that it uses moulds in the forming process. Freeform glass blowing is far superior in that all objects created refect completely the skill of the blower. I spent a year learning this in my youth and have carried many white-hot glass spark shrapnel scars from the experience. If you are serious then find a professional and volunteer a week of your time - you will be amazed at what is involved and may even be given the opportunity to see how much you cheeks inflate.