Lol, so that's what you mean.



It's quite easy actually. Just make a layer and draw the base color (yellow in this case), prefereably with sharp brush.

Then blur some of the edges a bit to make it look dynamic.

After you're finished, duplicate that layer and put it above the former layer. Apply gaussian blur filter on it to make glow effect. CSP and PS has the gaussian blur filter, dunno about SAI tho.

Lastly, you can make another layer on top of those two and change its mode to glow dodge or add(glow). Use airbrush softly with yellow or red color to amplify the glow.