Navigate to the releases section and grab whichever flavour or accent you want. Or, if you can't be bothered to find it, there is a convenient link here
Navigate to catppuccin-lb/dist and place the dist folder in your themes folder
[!NOTE] Don't know how to get to your themes folder? Simply type
.client theme browse
in your minecraft chat and it will open up the directory.
[!WARNING] This will override your current hud layout, including your positions of elements.
Simply type .client theme set catppuccin
in your minecraft chat.