Here we go!
At the beginning draw a circle for the head and other body guidelines. Then draw facial guidelines. One line in the middle of the circle should be enough, but if you want, you can also draw guideline for his mouth.
Deidara is dressed in characteristic Akatsuki's coat. He was rather slim, so the coat shouldn't be to wide.
Draw the legs, and then try to draw his ear and an outline of his headband. "Cut" the band in 3/4 of its length (the rest will be obscured by hair).
It's time to draw some hair. Deidara has quite distinctive hairstyle ;) Long fringe that covers half of his face and ponytail(?) in the back...
Now focus on the face. I think the eye is easy to draw. It looks like a half of an ellipse, with a curved line as a eyebrow.
Than sketch the mouth.
Try to draw hands now. Deidara had mouths on his hands, but they isn't hard to draw. It will be enough to draw a line in the shape of the eye for the mouth. Of course draw tongue or teeth.
It's almost done. Finish his clothes. Draw wide stripe along the whole length of the coat and some Akatsuki's clouds on the coat. When it's done, draw folds on the coat.
Last thing to do is to erase all guidelines and another useless lines.
et Voilà! Have fun drawing :)