How do I prune it to keep it small? It is 12 foot high and I would not like it to grow much taller.
If you are referring to the North American tulip tree (Liriodendron tulipifera) and not tulip magnolia, then you have the wrong tree in the wrong place. They grow to 50 meters in their native habitat. To hope to keep it 12 feet in height is unrealistic and would require extreme and detrimental pruning practices that would not be possible to sustain in the long term.
If the tree is a tulip magnolia then it still would be severely limiting natural form and beauty, but maybe more realistic than trying to miniaturize the tallest North American hardwood tree.