Select the sponge top bottle you wish to convert into a marker. Liquid glue and shoe polish both come in bottles that you can use for this purpose.
Remove the cap and unscrew and remove the sponge top from the bottle.
Empty the contents of the bottle and discard them.
Rinse the bottle and sponge cap thoroughly with soap and warm water until none of the original contents are left.
Dry the bottle, sponge tip and cap thoroughly with a clean towel.
Pour your desired color of permanent ink into the bottle. Fill just beneath the lip of the bottle so ink does not enter the sponge tip until you tip or squeeze the bottle.
Screw on the bottle's sponge tip and replace the cap.