Preheat the oven to 350 F.
Line the pan with parchment paper. Make sure to get it up over the sides.
Spread the marshmallows in a single layer on the parchment paper.
Place in on center rack and roast for about 5 minutes. The marshmallows will puff up and expand. When there is a slight golden brown on the top take them out. The bottoms are cooking faster than the tops so the bottom is twice as brown as the top. If you wait for the tops to be a dark golden brown the bottoms will be burnt.
Remove from oven and set aside. Let it cool for a few minutes. It's much easier to work with once they have cooled down. Then you can cut them to the size and shape you need. I learned this the hard way after I had started using them.