Once you have downloaded the book from the Smashwords Libary, plug your Kindle into the USB slot (small rectangular slot) of your computer using the cable that came with your Kindle. This makes your Kindle appear as a hard drive on your computer. After you purchase the book, from the book's book page click to download the "Kindle" format. Next, navigate to where you see the Kindle show up as a hard drive on your computer. Next, just drop the book's file (it should end in file name of .mobi) to the Kindle's "documents" folder. Then disconnect the Kindle from your computer and the book will be ready to read.
Smashwords purchase - How do I download books to my Kindle?