To convert a drawing you must have Adobe Streamline.
Take your drawing, scan it and after, increase the brightness/contrast.
When you have a clean picture, import it to Streamline and convert it to AI format.
After that, import the AI file in Photoshop.
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Or import the scanned drawing in Illustrator, as a BMP, and redraw above the original picture.
Use an elliptical brush with an angle of 45 degrees (or any brush setting that will make it more natural).
That's the tricks...
Good luck.