Take your marker of choice and scribble a little onto a none porous surface, take an aqua brush (I use Pentel brushes because I like their nibs / brush ends) and lift some of the marker ink off the surface, the more you lift the darker the lay-down color. It is better to lift small amounts at a time and build the color intensity and that's about it.
If you want to do a wash, say to go around the image as I have, ensure that your image is completely dry or you will end up with bleeding then lightly wet the areas you wish to wash, spritz some of your marker ink to make it quite watery then lay the color down starting where you want it darkest, usually closest to the image then blend it out.
Stamps: SU Summer by the Sea & Sassy Salutations (for inside greeting)
Paper: Water color paper, X-Press It blending paper, SU Melon Mambo & Midnight Muse
Ink: SU Melon Mambo, Stays On - Jet Black & Versa Mark - Watermark (for inside greeting)
Accessories: SU Stampin Write markers; Tangerine Twist, Blush Blossom, Midnight Muse, Soft Suede, So Saffron, Hello Honey. water color pallet, Pentel aqua brush medium, blending tool, X-Press It High Tack double sided tape, Eva Sheets craft foam.