Coat tile with alcohol. Drop little spots of ink onto tile. Allow ink to slide around a bit before adding the next color. The colors blend together the longer they sit. Cover surface completely. Use a paper towel to dab if too much ink was added, then recover with another color. Dry and spray with a coat of aerosol finisher to seal the colors. Dry and paint the sides of the coasters.
Craft Project: Alcohol Ink Tiles Part 1: Experimenting with Alcohol Inks
Alcohol Ink Coasters - pretty sure I have pinned this before but i love how they look and remind of those agate slice coasters (but cheaper!)