If you are still used to putting your mods directly into your .jar, or if you have a whole different jar client such as Obsidian, this tutorial is for you! Before we get started, here are a few things to remember. This tutorial is for the new Minecraft Launcher. It does not support cracked players because MagicLauncher only works with premium. Also, using hacked clients on most servers is not allowed.
First, mod your jar file of the version you want to use. For example, to mod 1.6.2 go to .minecraft > versions > 1.6.2 > 1.6.2.jar and put your mods in. For a client, delete 1.6.2.jar and put your client .jar file (Obsidian.jar for example) in the folder and rename it to 1.6.2.jar. There is no need in any instance to modify the .json file.
Next, download MagicLauncher and open it. Go to Setup, choose 1.6.2 and click OK. Now, when you use MagicLauncher it will use your modded 1.6.2.jar (or whatever version you are using). This works with clients like Obsidian. Although Obsidian and other clients may come with a .json file, that is not necessary and you should delete it – you do not need it.
Here is a video tutorial: