Search for calamity-inc.pluto-syntax-highlighting in the marketplace and press "Install."
Open the command palette (Ctrl+Shift+P), select "Package Control: Install Package", search for "Pluto", and press enter.
If you don't have Git installed, do so first.
In Sublime Text, select Preferences > Browse Packages... to open the "Packages" folder of your ST installation.
Open a console in this folder to run the following command:
git clone https://github.com/PlutoLang/Syntax-Highlighting "Pluto Syntax Highlighting"
You should now see "Pluto" as a syntax highlighting option in Sublime Text.
This project is provided under the Unlicense (dedicated to the public domain). However, it is based on https://github.com/LuaLS/lua.tmbundle, which itself is based on https://github.com/textmate/lua.tmbundle. All of these projects have different licenses. I'm not a lawyer, but from what I can tell, all of these licenses permit commerical use, private use, modification, & distribution while not providing any warranty.