If it is a metal door, they make great magnetic wreath hangers now. My son bought me one for Christmas that is very strong. It has a hook on it for the wreath and I just center my wooden dowel on it. For an apartment door, hang it high enough that it won't get knocked off from people with walkers brushing against it. As for hanging it straight, I would put the dowel/rod in the top for a straight edge. I would then attach a cord at the ends of the rod. I would tie a little loop in the middle of the string/cord for the hanger. This should keep it straight and secure. You might also think about making one wallhanging backboard for several button on seasonal greetings.