Fix it all

You can use a needle to sew the sides. If you don't have any, you can also pin them using safety pins.

But before that, we must insert our elastics or ribbons in each side.

If you don't have elastic bands, you can cut a length out of an old t-shirt or leggings. The cut part should be sufficiently elastic. There's no need to sew them.

Regarding the elastic bands going to the ears, cut 17cm / 6.7in and tie them tight.

To sew the sides, make a single stitch in a bias in one direction. Once at the end, follow the seam in the other direction. This way the two bias points will form crosses that will hold the pieces together. Pay attention to prick only the fabric on the back side and not the two sides of the mask at once.

Using safety pins to hold the last two folds is another possibility, instead of sewing them.