I am not sure what parts of this a caveman could do and what not... but:
If you google blogger templates or wordpress templates you can find a whole lot of free pre-made layouts, backgrounds etc that you just plug into the HTML portion of your blog design dashboard.
If you want a fancy header...like with photographs etc. after you chose your template, you open the source code for it and look for the header width of that particular template and then make a header in a photo or vector editing software at that size. The you put it in place of the header text that the template design is using.
Other things like widgets are built into the design elements... but things like buttons and other stuff have to be made in the same fashion as the header...
