Tap swipe the fuck out in a specific area.

Basically tap swiping is when you swiftly make a quick and small swipe with your pen while maintaining low pressure. By small, I mean VERY small and quick.

This is to be done in Paint Tool SAI.

Step 1. Use a drawing tablet. If you don't, this is a useless technique.

Step 2. Increase the brush size to a very large size. For 1600x1600 drawings, I prefer Size 500.

Step 3. Change the brush to a desired color and select the layer you want the clouds to exist in.

Step 4. Set the brush density to 35% and "Min Size" to 0%, and then go to Advanced Settings.

Step 5. Set "Max Dens Prs." to 100%, set "Hard <-> Soft" to 100, and set "Min Density" to 0%. Make sure "Edge Hardness" is 0.

Step 6. Set the checkboxes next to "Press" so Dens and Size are checked.

Step 7. Now, move the brush to the area you wish for a cloud to be, and rapidly tap swipe over and over in that area.

Step 8. Keep tap swiping until your cloud fits your desires. I would suggest some areas being more "dense" than others in the cloud so it looks realistic.

Now you got some nice, wispy cirrus clouds going around. Have a nice, god damn day.