For this tutorial, I wanted to do something simpler then my previous tutorials. This is not just a beginners tutorial though. This tutorial shows the simplicity and power that Illustrator possesses. In turn, this tutorial's techniques can be applied to many other elements other than fire.
Final Image Preview
First, let's take a look at the image we'll be creating. Below is the completed illustration to see what you'll be working toward.
Create a document that is 8.5 inches by 11 inches. Double-click on the Pencil Tool from the Tools Panel to bring up the Pencil Tool Preferences dialog. In the dialog change the Fidelity to .5.
With the Pencil Tool, draw a flame shape that is roughly 8.5 inches tall by 5 inches wide. Then fill it with black. Make sure that the Paths are closed by pressing Alt before finishing drawing the shape.
Draw another flame shape within the first one, making sure not to overlap the shapes (sometimes this causes the Blend not to work properly). Once you have finished drawing and closing the shape, fill it with an orange color.
Draw another flame shape within the previous one, and fill it with a yellow color. Keep in mind you want the path closed and not overlapping the other paths.
Draw another flame shape within the previous one and fill with a light orange.
Draw another flame shape within the previous one and fill with a light yellow.
Draw another flame shape within the previous one and fill with white.
Now that all the flame shapes are drawn, select all the flames and create a Blend by going to Object > Blend > Make.
Create a rectangle the same size as your document. Fill it with your first flame shape's black color. Then send it behind the flame blend by going to Object > Arrange > Send to Back.
All done! Wasn't that easy! Again with this tutorial, I wanted to stress the ease with which you can create compelling elements for illustrations. With these same techniques you can create realistic clouds, water, or whatever. These are great techniques to start experimenting with.