Simple.. after you download your desired skin code, log in to your blogger. Click 'Change Settings' then 'Template'. You'll se a box full of weird codes. Just delete those codes, block all (ctrl+A) and delete. Whooo.. disappear!
Open your downloaded skin code. Block all (ctrl+A) and copy (ctrl+C). Back to your blogger, paste (ctrl+V) your code into the box. Voila!! Preview it if you want. Just save your changes if you like it and republish your blog. Good luck!!