Building on the basic instruction in this post here is a more refined method. Note: it is better to leave the Black and White adjustment until after you have adjusted Curves...
Turn photo into a drawing. Simple method.
1, Place photo on New doc > duplicate and Select duplicated layer.
2, Filters >Detect edges
3, Layer >Invert (important) [Ctrl i]
4, Adjust >Curves. Drag low end of the curve line to the right
until all the lines you want to see are defined.
5, Drag the top end of the line to the left until most of the gray detail disappears
6, Reselect the duplicate layer
7, Adjust >Black and White and Voila!
8, Group it all together and Rasterise
If you want to use the line drawing as the top layer and place something like Watercolour brush effects underneath you can make the white layer transparent by choosing:
Filters >colours > Erase White Paper
This method is easier to understand and more effective than other methods I have seen demonstrated.