A lot of people try and paint tattoo flash without much grasp on how it actually works. Here's a step by step on how we paint tattoo flash.
First, get a rough sketch of what you want to do. We tend to use red pencil on tracking paper, but you can do it with any color on any paper. Some people draw directly on the water color paper and do the lines over top, but you're better off doing it on tracing paper as its generally easier to draw on the smoother surface, and it feels more forgiving if any errors are made along the way.
After you've gotten down what you want to paint, lay another piece of tracing paper over top and re-line it. We're doing all of this with a 1.0m micron pen. We do this just to get a much cleaner drawing to transfer over with all the extra details that you want to add.
After you've got it all lined out how you want it, cut it out to fit the paper size that You'll be painting on (in this case 5x7), and tape it to the back to avoid it moving around when lining it out.
Once you've done this, bring it to your light board and patiently and slowly line it out, ensuring it's as clean as possible. You can also tape your drawing against a window during the day or position a light behind the glass to shine the image through.
If you have any changes you may want to make, do it with red pencil so you can see it better, and just erase it after you've lined over it. In this case, I wasn't happy with the waves, so I redrew them with my pencil.
Make sure to WAIT for the lines to completely dry before trying to erase the pencil. If you try and erase it immediately after lining it, it'll smudge everywhere and ruin what you have so far. Here's my lines after erasing the pencil.
When you're ready to paint, make sure to ALWAYS do the blacks first. With water color, black paint won't run after it's dried when its hit with water, but colors will. So if you were to do the colors, then try and shade blacks, it would lift up the color and everything would run. So do the blacks first, shade everything out how you want, then move to the colors.
To achieve the black fades, or fades in general, we used a technique called spit-shading. How you do this, is you have 2 different brushes, and 1 cup of water with just clean water. With your color brush, you lay down a line of black paint. You then grab your water brush, dip it in the clean water cup, pull the excess water off of the actual brush with your lips (just pull it through your lips to pull out excess water so it doesn't drip but leaves the brush wet) and use that brush to smooth out the color. Just start at the edge of your black line, and brush it out until it's a smooth fade!
Once the blacks are done, move one color at a time. There have been many times where we've tried to switch colors, and forgot to wash the green off the brush and dip it in red and make a mess. So to avoid problems and make it go smoothly, I'd stick to one color at a time. Here's mine after I've done the reds.
And a few more colors added.
After I was finished, I decided to add some more detail until I was happy with it. I added some gray wash (an almost full cap of water with a small drop of black is what I used) and some red for the sunset. Once I wrapped that all up, I added my stamp and was finished! Just remember, practice makes perfect. Im still no where near where I hope to be, but every painting you do you'll notice them getting progressively better!