Welcome to the Bunnings Workshop community @chea. It's wonderful to have you join us, and many thanks for your question about constructing a mirror frame.
I've put together some renderings for you of a way to go about building these frames. I'd suggest using an 18mm MDF board as it will be relatively easy to cut and shape with a jigsaw. To get the larger sheet home, our helpful team in-store can cut it into a manageable size for you. Just remember to leave enough room around the perimeter of your chosen mirror for the curved frame. In the diagram below, I've based it off using the Living Elements 1200 x 350 x 5mm Flat Edge Mirror.
Take your mirror and trace the outline on two or three boards, depending on how thick you want the frame. Mark out the exterior of the curved frame shape around the glasses outline. Mark a second line for the frame's interior within the glasses outline on one of the boards. Make sure to leave enough room around the outside and inside the glasses outline. A salad bowl or suitable curved shape will be useful in keeping your guidelines consistent.
Cut out the frames from the boards with a jigsaw. Glue the two sections together with PVA glue. You can also screw these sections together if you wish. It would be beneficial to sand the frame smooth and fill any gaps between the boards with wood filler. You can then prime the frame and paint with your choice of enamel paint.
Place your mirror in the frame and hold it in position with glazier pins. You might like to also add a mounting wire to the rear for ease of hanging.
I hope that is of assistance. Please let me know if you have any questions.