Firstly, you want to make sure that the weight of you and whatever supplies you are using is going to be supported by the ladder. A burly man with a heavy tool belt can easily exceed the maximum limit of the extension ladder, thereby rendering it totally unsafe for him to climb. Once you are sure that you are the correct weight to climb an extension ladder, you will need to find a firm and flat piece of ground that you can place the ladder on. You then use the pulley system built into the ladder to extend the top piece as high as you need it to go. Then lean the ladder against the area you wish to climb to and then begin your ascent.
Picking the location of a ladder is perhaps the most important part of using this particular type of ladder. Since they are usually constructed completely from aluminum, you need to be absolutely sure that there are no electrical wires anywhere near where you are going to place your ladder. If an aluminum extension ladder were to come into contact with electricity, you could easily kill yourself or catch your home on fire. Also, do not attempt to overextend an extension ladder, as if you do, the ladder could break and send you falling to your demise in a matter of seconds.