Create a new canvas. Make it any size you want. Just remember, the bigger the better. That is as long as your computer can handle the rendering. I will use 1280x1024 with RGB mode and transparent background. Set your foreground color to black and background color to white by pressing D. Rename the layer Gasclouds. Now go to Filter>Render>Clouds. You will have something that looks like this...





Now go to Filter>Render>Difference Cloud. Then press Ctrl+F. Then use Image>Adjustments>Levels or simply press Ctrl+L. Set the Inputs Levels to 89, 0,52 and 200. You may have to change the inputs a bit, depending on how the clouds rendered in the beginning. This is what it will look like (or should ^_^)...







Create a new layer (Shift+Ctrl+N). Name it Saturation or whatever you want. Press D then use Filter>Render>Clouds. Then Filter>Blur>Radial Blur with 100% and Zoom as Blur Method and Good quality. Press Ctrl+F. It should look like this. Move it behind the Gasclouds layer. Change the Blending Mode to Color Dodge. Also change the Blending Mode of the Gasclouds layer to Multiply.






Now you are going to need a starfield. You can create a good and natural looking starfield if you follow the instrucations given by Greg Martin's tutorial... Once you have created your starfield duplicate it to your nebula canvas. Put it under the Gasclouds layer.

Download the starfield I used here...
starfield(1).jpg