Hard Way:
1. Download the skin.
2. Go to minecraft.net/.
3. Login
4. Go to "Profile".
5. Scroll down until you see the words "Change How You Look In Minecraft"
6. Click the "Choose File" button.
7. It will open an "Open" tab.
8. Find your skin.
9. Click on the skin and then click the "Open" button.
10. The "Open" tab will close. Click the "Upload" button which is found in the "Change How You Look In Minecraft" Section.
11. Run Minecraft.
12. If you play offline, you will not be able to use your skin.
Easy Way:
1. Click the "Change My Skin" button above the Download button.
2. Login.
3. Click the "Change" button.
4. Play Minecraft.