What's a shiny inexpensive metal used to make mirrors?

Get a piece of silver paper, cloth, etc. Something that's rather shiny and reflective. Cut it to the size you need. Aluminum foil works well for a prop-like mirror.
2
Glue it to a piece of wood or stiff paper. Whatever type of glue you use, make sure you can spread it on evenly. Be careful if you're using a hot glue gun -- burning yourself with glue isn't a fun part of a DIY project.
3
Get a sheet of glass and put over the paper/cloth. This top layer makes it look a lot more professional and keeps the mirror from getting damaged or torn. The glass on a cheap frame is ideal for this step.
4
Design an appealing frame. This could be cardstock, cardboard, fabric, or any material that can form a border. Add glitter, ribbons, beads, or any embellishment you have available.
The easiest way is to use a real frame and spruce it up how you like. Find a cheap one and spray paint it, paper mache, or otherwise make it your own.