Start with two white cardstock. No white cardstock? Hate to say this but this is true, sometimes I use the back of patterned paper (which I don't use or like or been in my stash forever)
Apply a fairly thick coat of gesso on one of the cardstock so you can see the texture when you are done. No precise brush strokes necessary, just play with it.
Using the other plain cardstock, cover the other cardstock (the one with the gesso)
Press, rub a little bit.
Open it slowly to get more texture on both side of the cardstock. Don't wait to long to open the cardstock, because the longer you wait, they might stick together and it will not easy to open.
Let it dry. Apply paint, again just play with it, have fun painting.
Sometimes, I use wet wipes to wipe off the paint in case I apply too much or if I want to have uneven effect and interesting looks.
If you want to add more colors, just let it dry first before you apply the next color.
Again, rub and play with the wet wipes.
And Voila! It's done!