Here is an image of how you will tie and tuck in the mask.
Determine if the mask is sticking up anywhere and mark with chalk. We have 5/8" seam to play with, so you can adjust certain parts of the seams by sewing more or less of a seam.
The seam that presents the most trouble is where the top meets the back. It tends to pucker up, but can be solved if you stitch again, leaving more seam allowance.
After you've adjusted the mask, Put the ears in place while wearing the mask and carefully pin them where you would like them to be. (Don't stab yourself in the head with the pin!) It may also help to have a friend mark along the edge of the ear with chalk. Remove the mask and topstitch the ears in place.
Now you can try on the finished mask.