We're going to install tact switches where the current button contacts are located. This will activate the tact switch when a button is pressed on the NES controller. Drill small pilot holes in the center of each button to ensure an accurate location. Drill them out to a larger size and file each hole to allow a tact switch to fit.
The depth of the tact switch alters the feel of the controller. Install the switches level with the PCB and the controller will have a softer mushier feel. Install the switches so they protrude from the PCB and the buttons will be firmer. I chose to allow them to protrude around 1mm. Install all of your tact switches and add some hot glue to hold them in place.