How to install Users Online Stats

how to install Users Online Stats?. How many people is online at your blog? you know that or don't? If you don't know how many people is online at your blog, you can use a free tracker to know it. That is much site provide this service. Once of them is www.histats.com. If you use them tool, you can know how many people online at your blog, how many people was visited to your blog, and any