You're a beginner here it seems, so start easy. Build a rectangle of cloud, then dig out a rough section in the middle (easily done by clicking and holding with the Laser Drill and waving it around) and fill that with dirt/other material you wanna build your base on/your base, or a room of it. As for the cloud, well, fly around it, and if you have the Drill Mount and I hope you do, it gets a lot easier; fly around under the cloud and try and turn the shape into somewhat of a hemisphere. If you cut too much, just add a few layers of cloud back. Once you have it somewhat sphere-like, get down there and start adding little block patterns. An L here, a reaching out 4x4 chunk, one or more missing blocks. This is mandatory. I discovered it in the start of me building 22 floating island homes.
Try and make it random, and if it helps you try to imagine that the details you're adding are side effects of wind around the cloud.
As for the rockets - I'll just tell you to see if having them dig in a bit into the land above looks good. Or an extended pillar built from them, smth like that. Also, I'd suggest digging the cloud around the, err, turbines? exit ports? metal parts with propulsion characteristics? hot stuff? Dig the cloud directly around as if it seems it was blown away. Make a little cone or smth.
OOOR skip all this above and as shobiwan77 said, imitate floating islands.
Right, that's all I can do for you from the getgo. If you have questions just ask, and good luck with your building!
