Select a dark background colour
Duplicate the monoline brush. Select properties and increase max size to halfway
Draw a large circle
Use the automatic selection tool and tap the inside and edge of the circle
Select adjustments and Gaussian blur, then slide until about 15-20%
Keep it selected and add a new layer
Increase the brush size and add highlights around the edge of the bubble
Use Gaussian blur again and blur to around 25%
Keep it selected and add a new layer again
Select white and draw the highlights
Use Gaussian blur again to blur
Merge or group layers if desired
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