See how to put layers in a straight line on your canvas. If you have small layer shapes like circles, you will need to make sure the layer border is reduced to the edges of the shapes (using autocrop).
Accurately straighten a picture that is a little tilted. Use the measure tool to find the correct angle that you need, then rotate the picture by that angle.
Scroll down to the section "Flip An Image"
on this webpage:
<Image> Tools > Transform Tools > Flip