Its very simple and easy.
NOTICE: I in no way own Pokemon, Gameboy, or any other trademarks . Those are all property of Nintendo, Game Freak.
I am also not responsible for any damage or injury which may occur while attempting this.
Proceed at your own risk.
Step 1: Materials
Screwdriver 1.5mm or Trigram scredriver
-Pen eraser,not in photo(http://www.aliexpress.com/item-img/Faber-castell-7016-40-fountain-pen-eraser-pen-fountain-pen-ballpoint-pen-sand-rubber/735798383.html)
-Bottle cap .
Step 2: Unscrew.
Then carefully separate the gameboy.
Now in the back part remove the metal plate and place the 4 screws in some safe place.
With the help of the screwdriver apply some pressure in the battery metal plate.Be very gentle.
Step 3: Front.
Remove the screws.And clean with the cotton swab the speaker.
Step 4: Bath Time!!
Use medium hot water with a little of detergent of dishes.
Then dry every cornerwith a towel ,DO NOT USE A HAIR DRYER.
Front side water+cotton swab+Patience=Sucess