The horizon in this photo is the only hard edge. All the clouds are soft. If you were painting a version of this scene, therefore, the first step would be to wet the paper.
You may have found yesterday that mixing your colors took quite a while, and by the time you were ready to apply the first layer of paint the paper was dry. If you intend to make soft-edged shapes and you get hard ones, stop painting as soon as you notice. To be sure, you can make a very small mark toward the edge of the paper to see whether the paper is still wet. If that test mark comes out soft, carry on. If it comes out hard-edged, stop painting. You are in charge, not the paper. Remember, you can re-wet the paper once it's thoroughly dry and create just the kind of edges you intend.
I wet my paper on both sides so it would stay wet longer. It helps to think of the wetting as a process of getting some water into the paper, not just on the surface. I go over the sheet lots of times with a BIG brush, in both directions. Put a little extra on the edges. They dry faster than the middle. After the clouds had all been painted the paper was still wet, so I dried it thoroughly so I would get a hard edge on the horizon.
Here are a couple more good skies to consider.
About those colors:
My one sentence theory of color mixing goes like this, "To get the color you've got to look like the color you want, add red, yellow or blue".
For this exercise, limit your palette to one each of the primary colors. That is, one red, one yellow, and one blue.
Please bring in all your attempts. Part of what we're practicing is diagnosing what went wrong in a failed study.