1. Create a new 375x375px document. Fill it with any color, then give it a Gradient Overlay layer style:
And a Inner Glow:
Which should give you this:
2. Create a new layer, select the brush tool, and choose the Smoother Round Bristle brush from the Thick Heavy Brushes set. Increase the diameter to a large number like 1000-1500px:
Draw some random splotches with the brush, using any color (I used white).
3. Give the brush layer a Outer Glow layer style.
For something roughly like this:
Drop the fill of the brush layer to 0% - this will get rid of the fill color and leave just the glow effect. Since you used the Color Dodge blending mode in the last step, nothing will show up where the background is black.
4. Duplicate your brush layer, then go to 'Edit > Transform > Rotate 90 degrees CW'
Duplicate the new layer, and rotate this layer 90 degrees clockwise, just like the last one:
5. On a new layer, create a circle around the plasma with the Ellipse Tool, and give it this Inner Glow layer style. Drop the fill of this layer to 0%, just like the brush layers. You might need to adjust the size of the glow a little.
Which should give you this lovely plasma ball:
6. If you have my luck, this will be the step where Photoshop crashes and you did not bother to save anything. So I followed my tutorial and made another. This is a higher quality jpeg, and I used different brush strokes this time around. It goes to show that the results will vary, so if you don't like the look of your plasma, try experimenting with the brush layer.