the provided launcher file (often an .exe or .bat file).

Minecraft 1.8.9 requires . To make it truly portable, download a "JRE 8" folder and place it on your USB. In your launcher settings, point the Java path to the executable on your USB drive rather than the one installed on the computer. Staying Safe While Downloading

Since Minecraft typically saves data to a hidden system folder ( %appdata% ), you need to create a dedicated space to redirect that data.