The most straight-forward way to install the bundle and to keep it up-to-date is through Package Control. See how to install Package Control on Sublime Text.
If you want to develop on a forked repo, you can clone it into the Packages folder:
cd ~/Library/Application\ Support/Sublime\ Text\ 3/Packages git clone https://github.com/<fork author>/haxe-sublime-bundle.git Haxe
cd ~/.config/sublime-text-3/Packages git clone https://github.com/<fork author>/haxe-sublime-bundle.git Haxe
Using git bash
cd /c/Users/<username>/AppData/Roaming/Sublime\ Text\ 3/Packages git clone https://github.com/<fork author>/haxe-sublime-bundle.git Haxe
Restart Sublime Text.
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