How to remove the white from drawings and keep a pencil texture to make a 'Digital Collage'.

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Draw your drawing then scan it in.
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Double click "Background" layer to rasterize the layer.
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Ctrl shift + U to desaturate and Ctrl + T to transform and line up the image.
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Ctrl + L to adjust the levels. Push the darks up and the lights down to even out the image.
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Select > Colour range, click the white area and change selection preview to Black Matte, then adjust fuzziness until the darks look full. Delete the selection. Ctrl + D to deselect.
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Triple click the layer then colour overlay, then you can change colours on the fly.

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Now you can move elements around, seperate them into layers, resize and change colours until you have a nice result.

I hope that was clear enough. Enjoy!