This is how I did some of the ATC's for the exchange on Wet Canvas. Even though I tried to get them similar to the ones I did for the Exchange, they never turn out the same the second time.
heavy water colour paper. 300 lb is best for less buckling when wet. Where you have a rough and a smooth side, smooth is perhaps better for ATC size.
Ink - for the skies in these I used Parker Quink blue because it separates into blue and ochre.
Calligraphy acrylic inks – red, yellow, sepia and green.
Preparation. I put on art masking fluid for the shape of the trees and horizon. I drew pencil lines roughly where this was going to go so you can see but this is not necessary as the masking fluid is yellowish. Wait until dry.
Then I dampened the back of the water colour paper to stop curling. On the front I wet the sky area, some of these were wetter than others. Some had more ink on than others. Where a heavy dab of ink was in a wet area – (not damp – wet )- it was able to spread and this left the yellow-ochre tinge. I helped the spread in some cases by tipping this way and that.
Don’t try to paint cloud shapes just dab along the top and perhaps in a couple of other places. In # 3 I drew some lines on the right and dabbed a little in the middle. The other sky areas and clouds are formed by the paper not being uniformly wet.
Try to keep the application of the ink on the top of the picture so it is thinner towards the horizon with the spread. You can spread a thin wash of blue over the masking fluid tree trunks. In #4 and #1 I didn’t because I will wash with another colour. Wait until dry.
Rub off masking fluid. This will also mostly rub out any pencil lines.