Create Ringtone
Download the MP3 from Soundboard.com that you want as a ringtone and add it to iTunes. Once added, right click (control + click) or from the Advanced menu item on the menu bar select the "Create AAC version" option.

Rename and Add the Ringtone
Once the file has been created, again right click (control + click) and select the "Show in Finder" option; on Windows you should click "Show in Windows Explorer". Now rename the file and change the file extension from ".m4a" to ".m4r". Remove the MP3 and AAC version from iTunes, and drag the newly renamed file into iTunes. The ringtone should now show up under the Ringtones section of the Library in the left hand sidebar.

Transfer and Test the Ringtone
Now that you have your ringtone, go to the Ringtones section of the Library in the left hand sidebar and drag the newly created ringtone to the iPhone you want to use it on, just as you would an MP3. You should now see your ringtone on your iPhone, under the Settings / Sounds / Ringtone menu. Select your Ringtone to preview it, and you are all done!

Download Track
Download the MP3 on Soundboard.com that you want to use as a ringtone.

Add Ringtone
If you haven't setup your phone to use your SD card as a storage device, go to your phone Settings / SD and Memory Settings and select "Use SD card as storage device". If you don't already have a "ringtones" directory, crate a new directory at the root of your SD card named "ringtones", you should also make the folders "alarms" for the alarm clock and "notifications" for text messages and other notifications. Add your new audio file to the appropriate folder.

Test the Ringtone
Select ringtone to preview and you are all done!