I'm using the real launcher
I already have Forge installed
Okay! This is easy.
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Ensure that the Minecraft Launcher is closed
Run the LiteLoader installer, you will be see the following screen
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If your .minecraft directory is not shown in the selection box, then click the … button and browse to the correct location
In the Extend from box, select the version of Minecraft Forge you want to use
Click OK to run the installation
Next question: Do you want to use any other tweaks? (eg. Optifine or ShadersMod)