How do you make paper mache shield?

You could use a garbage can lid for a mold for a round shield. Use modeling clay to cover the handle and make the lid smooth.

You could also use a saucer sled for a mold if you want a really big shield. If you use one of these, you can get a smoother front surface by molding the shield on the side of the saucer you sit on. Lay the first layer of strips radiating outward from the center

1. Lay the next layer of strips at right angles to the first.
2. Alternate like that as you build up a half-dozen or so layers.
3. After it dries, pop it off the mold and trim the edges.
4. Lay more strips around the edge to give it a finished look.
5. Attach the arm straps of leather, rope or cloth by cutting slots through the shield and hold them in place using staples, glue or brass fold-over brads.
6. Lay a few more strips of paper mache over the straps on the front side to cover them.
7. Glue cardboard cut-outs to the front surface to create decorative details.
8. Let that dry and decorate it with paint.

Don't rush the project. It needs to dry well before you paint it. It might take a week to do it up right.

See the link for some authentic round Roman shield designs.