All you have to do is install webmin according to the installation instructions you'll find in the INSTALL.txt file after you download webim.
Then Generate the button code HTML from the webmin administration side... and post that HTML onto your website. If you want it in a side block, you need to create a new module for custom HTML and choose the side you want the block to appear on(if your template allows).
Then paste the HTML into the editor of the module.