Super easy!
The old gasket simply pulls off from around the inside of the door stop area. To install the new one I just started in the corner on one side and worked my way around to the other side. I just used my hands to work it in tightly but you could use a flat screwdriver or similar to ensure a tight fit, just be sure not to pinch or puncture the new one with any tools.
It also came with a new door latch which was awesome and very easy to replace. Unscrew two screws on the underside of your counter to be able to pull the unit out 2-3 inches, compress the latch locks and slide it out. Replace with the new latch and reinstall the two screws that hold the unit to the counter.