You can do them a few diffrent way's The easyest Ill talk about first. Hella makes these lights. They only make them in a few diffrent colors, Red, Blue, Green, White. They cost $25 each you can buy them at West marine, Im sure you can find them cheeper if you look around. BUt all you have to do is cut out the bottom of the cup holder and then drop the light into the hole.
The second way is you buy some 4 LED light pucks.
Take them apart. Then drill a hole in the bottom of the cup holder so the wire cam come threw. Then glue the 4 LED light pucks to the bottom of the cup hoder. Now the fun part. You will need to add or fill the bottom of the cup holder with casting resin so you can create a flat surface. Tap plastic sells casting resin it mixes clear.
You will need to put the cup holder in somthing that holds them flat so when you pour the casting resin into the cup holder so it drys level
The casting resin will create a clear flat surface that drink will sit on.