Looks like a variation of a Nuke egg.
https://news.orvis.com/fly-fishing/video-tie-nuke-egg
Tie in the yellow tag ( yellow thread, floss or similar) before adding body (pink colored egg). Clip the veil short as opposed to leaving long as in video.
Hardest part will be getting the egg right. As simple as an egg fly is, they are not the easiest to tie properly without alot of practice. Mcfly foam is a big help as opposed to egg yarn, though both are acceptable.