Try the shoes on with the thickest socks you might wear with those shoes.
Measurements should be taken while you are standing
Have both feet measured. Most people have one larger foot. Fit to the large one
The longest toe should be a finger's breadth from the end of the toe box
The counter around the heel should be reasonably snug
The forefoot should not be crowded
Foot should not budge over the sole
Don't worry about the stated "size" of the shoe. Worry about the "fit." It doesn't matter that you wore a size 5 when you were 22 and the clerk is telling you to try a size 9. Only Cinderella always wears the same size shoe. Your foot size will change as you grow older. Also, sizes vary between brands.
Don't buy a shoe if it feels too tight. It will NOT stretch to fit your foot.