1. Rotating 180°, 90° clockwise, 90° counterclockwise, flipping horizontally or vertically is easy: select Image - Rotate, and then the appropriate direction. Rotating just a few degrees is a bit trickier.
2. In both images, choose the same two points to check for angle. Using the measuring tool (see "Sizing") draw a measuring line from one point to the other.
3. Check the angle of the lines under "Info."
4. Calculate the difference between the angle in Image 1 and Image 2, and determine whether Image 2 needs to be rotated clockwise or counterclockwise to match Image 1.
5. In Image 2, select Image - Rotate - Arbitrary, type in the degree difference, click on "CW" (clockwise) or "CCW" (counter-clockwise) and click "OK."
Rotating the "canvas" means that the entire image is rotated, including all layers that might be added to a file. The computer interpolates pixels along the edges to fill in the space that would be left blank by the rotation process.
Rotating individual layers will be addressed under Layers.