the colour choice is important.. make sure you have the brightest/most vibrant selection of colours possible..
then you need a steady hand and patience.
I drew the basic outline of the letters first then on different layers for each colour added the "brush strokes" until it formed something that i thought looked good.. then i sharpened it, added a background for some depth and added some lighting effecs. I added the splattered paint effects after too from making my own brushes.
I don't really have time to make a tutorial but i hope this will help.