Open up your image that you want to trace, then goto "layer > new > layer from background" rename this layer "original" then create 2 more layers one called "white" and the other one called "lines" your layers pallet should look like this.
Now select your "white layer" and fill it with the color white by using the paint bucket tool. Once filled hide that layer, i do this so we can see how our line drawing is going by hiding and unhiding the layer.
Select your original layer and goto "image > adjustments > brightness/contrast" and brighten your layer by about +50, this makes it easier to see the lines. (well i think so anyway).
Select your brush tool and select a "1 pixel" "black" brush. Now select lines layer and then the pen tool and just start tracing. (zooming in will help for tricky bits)
Here is the shoulder from the picture I'm am tracing, notice the marker points created by the pen tool.
Don't try and select everything all in one go, just do bits at a time. Once you have traced a bit of the image right-click and goto "stroke path".
This will then create a 1 pixel line where your points were, then after that right-click again and goto delete path.
If you "unhide" your "white" layer you will see the line you have created.
Once you have done the whole image you should have something like this.
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