How to make a flame that looks real enough...but not so real it looks like you cheated somehow!
Firstly, pick a shade of Orange that suits you (Red or Yellow could be used to, but I'd go Orange if you're making it look real). Then, with "pen" drawing tool and 100 brush size, make a circle without moving your cursor too much, you don't want the circle too big. (The Black background is optional...you can choose any colour or leave it blank. Please note that depending on your background colour, effects your flame in the end.)
Now, the effects come into action. Use the smudge tool, brush size 16, opacity 128. From the bottom of the Orange/Red/Yellow spot, at medium speed sweep up the spot a few times. I have done it five times. Also for better effect, the closer you are to the sides, you can curve the smudge in so it still keeps the ball shape.
Now, using the smudge tool still, smudge through the areas where you didn't smudge in the previous step. When I did it, my sides looked like they were back to how I started, so I smudged the sides up a bit more. You may want to smudge a whole lot more of the flame.
Now comes the fun part. Experiment a little with the smudge tool. Sweep through the flame over and over until you get the shape you want. You may end up redoing the shapes. I came up with several flames.
Just so you know, these are obviously only four of millions of possible shapes. Also depending on the shape you want. For example-If you want a candle light (like this bad drawing to the left), Curve the flames with the smudge tool toward the center top of the spot you had. Also...if you use a big dot at 100 brush size, you might want to use a bigger brush size than 16 for the smudge tool. (Please, try to ignore my horrible candle...)