The SNES Lite kit is a simple mod to add a power LED to the model 2 aka "Jr" Super Nintendo/Super Famicom.  The kit consists of an LED mounting board and a 3D printed light guide.  The first step is to drill a hole for the fiber optic portion of the light guide, located in the small indentation directly next to the embossed "On" text for the power switch.  The hole should be as close to 1.5mm as possible.  I suggest using an undersized drill bit if possible, and then widening the hole to size.  You can check the hole size by inserting the fiber optic cable into the hole from the top of the case.  It is VERY IMPORTANT that the hole is drilled perfectly vertical, and it may help to drill the hole while the two halves of the case are assembled together.  Otherwise, it is very easy to end up drilling at an angle due to the curve of the case.  While there are no components located under this spot inside the case, it might be a good idea to remove the main board and reassemble the empty case halves before drilling.

Correct drill angle

Incorrect drill angle

Once the hole is drilled, simply insert the 3D printed light guide into the underside of the top shell.  This step may be tricky the first time, as the fiber optic cable may not line up exactly with the hole you drilled.  If this happens, take your time, insert the light guide at an angle, and press the fiber optic cable towards the hole with a small screwdriver.  Once you've managed to install it once, the pressure should pull the fiber optic cable into place such that it is much easier to install in the future if you choose to remove it.  When installed correctly, the light guide should sit very flush with the inside of the case.  If it doesn't, you probably haven't gotten the fiber optic cable into the hole correctly.  DO NOT TRY TO FORCE THE LIGHT GUIDE INTO PLACE.  If the fiber optic cable is lined up correctly, it should slide easily into place, if it doesn't, don't just press on the light guide harder, you may end up breaking something (most likely the fiber optic cable) if you do.

Once the light guide is installed, prep the SNES mainboard by removing a bit of solder from the 3 pins shown on the bottom of the power switch.  Don't remove all of the solder, just enough to allow the LED mount to sit flush against the main board.  Once you've removed the solder, set the LED board in place and solder it down.  Be sure to add enough solder to the joints to flow down to the main board and replenish the original joints, or you may have power issues.

Once the LED board is soldered, you may want to test that it works before reassembling everything.  You may also want to test fit the main board against the top shell to ensure proper alignment with the light guide.  Once you are satisfied that everything is working correctly, simply reassemble the console, and you're good to go!