This will be for people who are somewhat comfortable with installing mods, if you need more specific help ask in comments (This is also how it is done on Windows, Mac will be slightly different and I know fuck all about linux OS's so goodluck)

Note: This is for 1.7.2 only, make sure you have it on your computer before starting.

1. Go here and click the ad.fly link to download the rar file.
2. Next go to your .minecraft folder (%appdata% in run[winkey R])
3. Find the versions folder inside your .minecraft folder
4. Look for 1.7.2 and make a copy of it
5. Rename your new 1.7.2 folder to 1.7.2modded
6. Open 1.7.2modded and inside the folder you'll find 1.7.2.jar and 1.7.2.json
7. Rename those two files to 1.7.2modded.jar and 1.7.2modded.json (Up to this step your folder should look like this.
8. Now you need to open 1.7.2modded.json with notepad (Right click ---> open with..notepad)
9. Look for:"id": "1.7.2",
10. Rename it to look like this and save it.
11. Open your .jar and drag the refnsk folder and bah.class into the .jar and press yes to any "add and replace" dialog box. 11.b. Delete the META-INF folder (VERY VERY IMPORTANT STEP)
12. Close the .jar and open minecraft and click "New profile" in the bottom left corner.
13. Name it anything you want to remind yourself it will be the modded folder.
14. At the bottom of the profile creator, you'll find a drop down box called "Use version:" Change this to release 1.7.2modded (It will look like this
15. Save and click play! You should be all ready.
16. Save profile and you should be ready.