It varies slightly depending on which specific Instax camera model or MIni Link smartphone printer you're using, but you need to open up the film and also the back of the camera and simply insert the entire black cartridge into the camera facing down, or so the little yellow tab is facing upright and aligned with the tab on the back of your camera. It's really really easy and takes about 4 seconds.
The first image you print after loading the film cartridge will be a blank, as the included black card slides out of the top. Be sure to NEVER open the back of your Instax camera if you have unused film still left in there, as these will then become overexposed to natural light and simply turn out white with no image.