Um, why not just use the ruler tool to mark a straight line, then Image > Rotate > Arbitrary > OK? This will rotate it perfectly.

Skew is if the page was photographed instead of scanned, and there is (usually keystone) deformation from not holding the phone perfectly flat.

but.. GIMP D:

help, I'm stuck on ms paint

W-why I can't spot any difference?..

@Skankyslack I gave you Photoshop, use that instead :whip:

@Xunder help this guide doesn't use GIMP, what do?