The first step is to paint the single sheet of watercolor paper with the background. I use Arches watercolor pads. I love these pads because all of the sheets are bound together. You work on the top sheet and because they are bound, they dry flat without warping/buckling.
You don't have to have a lot of talent to be able to do this type of project. You are simply providing a watercolor wash. Use the colors that you like or that you have in mind for the project. I use Dr. Ph. Martins concentrated watercolors in the bottle. In essence, you just play--almost like finger painting. You get your brush wet, put on some color and move it around. The only recommendation is that you start light and work to dark. Your darkest color should be the one you use the least. With my watercolors, you more water you use, the more transparent (lighter) the color.
Once the paper is dry, you ModgePodge the whole sheet--while it is still attached to the paper block. You can use a brush. I use a plastic shopping bag from the grocery store that I scrunch up into a ball. I dip the ball into the ModgePodge and tap away on the painted paper until the entire surface is covered. You only need one coat.
Let that dry and cut the paper to size. I use the 18" by 30" size paper so I can get about 15 4by 6 inch postcards.
The beauty of the project is that you never know what you are going to get until you cut your postcards. You can think that you have ruined an entire piece of paper by adding a color but when you cut it down to size, it doesn't look bad. Similarly, if you don't like a portion of the page then you can only use the portion that you like.
I thought that I had ruined this sheet by adding a huge swatch of red on a background of blues and greens. I wanted a dramatic color to offset the more subtle shadings of the blues and greens. Now that I see it on the smaller scale, I rather like it.
Four of the basic watercolor cards.