First off, I suggest you copy files from the base folder to your template folder, following the same folder structure of course, because your template overrides the base files. So if you have the same left-col.phtml file in both base and template directories, Magento will read the file from the template directory only.
Now the easy way to do this is to get an extension called ZBlocks from Aheadworks. Very cool extension that basically turns Magento into a Joomla like module structure.
So if you want to work just through Magento, you’ll can create a new static block in your CMS and input your html there. Next, you’ll need to call that static block into your template layout files in the column you want using the following: