Because boots don't have laces, proper fit is all about the instep.
Boots should feel snug in this area, but not tight.
If the instep is too tight, go for a wider or bigger size.
The widest part of the foot is called the ball (it's where you bend your foot when you walk or run).
Make sure the ball of your foot lines up with the the ball of the boot (that's the widest part of the boot where the steel shank ends).
If the boot is too short, your toes will be forced into the toe box.
Boots should slip slightly in the heel. Boots are built in such a way that there's nothing to keep your heel from riding up a bit.
As you wear the boot, the sole will become more flexible and most of the slippage will disappear with time.
If the instep is too loose, the boot will slip too much, which means that you need a narrower or smaller size boot.