The first thing you have to do is choose the righ paper, it has to be thick and with pores (if you have a watercolor paper is the best option if not choose one according to this two characteristic)
Y por último para creer este efecto es muy importante usar mucha agua (tampoco demasiado) esto esta mejor explicado en el video.
The second thing is choosing the righ brush for the righ paper size, you will need one really big to wet the paper with water at the beggining and then another smaller one to add the color. Making the cardstock wet is the essencial for this technique!
As well as using "a lot" of water when applying the watercolor, more water than the regular watercolor but not that much
Y finalmente después de todo esto este sería el resultado:
And finally this would be the result:
Here is a video from the beggining to end, is much easier to understand! (sorry is in Spanish but is worth watching to see the technique)
You can show me your results in my Facebook Page