You can find simple, easy instructions here:Compender: A Simple Blogger Tag Cloud Widget
It's the only one that I've found that is so fabulously simple to use. It took me all of about 15 minutes to get it installed and all the variables adjusted to my liking.
Definitely read the comments - skim over to the ones that mention adjustments to the html. They tell you how to change the number of times a tag shows up, the font size, etc. I made a couple of adjustments myself to color and text alignment, so feel free to ask if you'd like to know. :)