Make sketches of the tree and the body posture (axis) - use a google image (you'll find images on clothing also) or ask someone to be your model, then add shadows, highlights, cloths, accessories, tree, landscape...a horse if possible.
If you are skilled, draw your cowboy in more complex angle.
- neck bent, hat on the face :)
- face shadowed or completely covered
- torso: the axis must not have too much continuity with the legs, everything must be relaxed
- arms crossed upon chest or relaxed near body, holding bottle or gun
- legs stretched in a relaxed position or bent one on the other :)
- typical plaid shirt - bit wrinkled -and hat, traditional leather pants, star if sheriff, boots
- if comics, you can add "zzzzz" or a toothpick coming out from his mouth :)
Goodluck, hope i could help...