When making a card blank I cut the card to the total size I want, score down the middle, fold it then, with my guillotine put the folded side up against the end, trim one side (which will be the top), turn round, and trim the other side (which will be the bottom) then trim the side opposite the fold, phew! Usually results in a "square" finish.

The other thing I now do is trim a very small strip off the r/h side on the back flap to make opening the card easier - yes it really works.

If making a card with lots of heavy embellishments of the front I trim, with a craft knife and ruler, a very small diagonal cut on the bottom of the back flap going up from the middle fold to the outside edge, makes the card tip back slightly to compensate for the weight on the front. IYSWIM.