Following are a few ways you can create cloud effects in your paintings. #1 will be if you have a lot of clouds, but also a lot of blue sky, and need to mask off the clouds.(USING NO MASKING FLUID) #2 will show what to do if you have a lot of sky and only a few clouds. And #3 will show mostly clouds with a few peeks of blue sky.
#1 Lots of sky/lots of big cloud formations.
If you want to mask off clouds so that you have some freedom to wash in lots of sky, here is one way. First, tear some tracing paper in the general size and shape of your cloud. Rip strips of masking tape and tape the tracing paper to the painting ripped side out. (This is to reduce the hard-edged look that masking things off often gets.) Leslie Macracken (who is BTW a man) suggests using a knife to cut the edges of the tape to the cloud shapes. I didn't care about the specific shapes enough to try that.