Fold it over inside out and just sew up one edge. Then turn it right side out.
If you do have two different types of cloth you’ll have to sew both sides.
After that, follow the steps in the video to fold and pin the mask fabric. In the video they suggest using ribbon for a tie, but I think elastic works better. I’ve tried a few different types, but I think thinner types work better. I got this elastic at Coles (grocery store in Australia). If you use this thin type, tie a knot at each end so you can sew it down securely. Just sew around the knot.
Next, fold over the edge of the fabric and sew along the edge.
Do the same with the other side of the mask, and you are done!
Wearing masks isn’t the only thing you should do to help reduce the spread of Covid-19. If you’d like to learn more, take a look at the blog post written by Dr Erin Bromage.