You have donwloaded it, right?
Now let's set up the environment for the ease of use.
Linux
tar xvf calm.tgz --directory=$HOME/
echo 'export PATH="$HOME/calm/:$PATH"'>> ~/.bashrc
macOS
Open the downloaded DMG file
Drag the .app to the Applications folder
Trust me and de-quarantine Calm.app
xattr -d com.apple.quarantine /Applications/Calm.app
Eject the DMG and run Calm.app
It may take a while for the first run.
Add command calm
to the PATH environment
echo 'export PATH="/Applications/Calm.app/Contents/MacOS/:$PATH"'>> ~/.bashrc
Windows
Right click to Extract All
Move the extracted folder to C:\calm
Add C:\calm
to your PATH environment variable
If you are using MSYS2 or Git Bash, run the following code:
echo 'export PATH="/c/calm/:$PATH"' >> ~/.bashrc
On Windows, it's very likely the system will stop you from running the software, since it's not verified by Microsoft. If you have problem to run it, please find a way to bypass SmartScreen.
Now you are all set, enjoy.