Do the middle buckle (if you have a 3 buckle boot) or 2 middle buckles (if you have a 4 buckle boot) first. these are the most important buckles because they hold your heel down in the boot. from there tighten the top buckle, then the toe. The toe buckle should only have enough pressure to keep the buckle buckled. Any tighter and you are just cutting off your circulation with no benefit.
In addition, use the two finger rule. If you cant buckle a buckle with your index and middle finger, it's too tight.