Optifine capes

So basically I have a friend with a cool optifine cape design, however, I don't know how to make it, is there a website, that tells me how to make HIS cape? not some website that has a bunch of capes i have to look through, but tell me how to directly make HIS cape/copy it

If you want to make your own custom ones:
https://www.needcoolshoes.com/banner



Else, you can go to planet minecraft, look through their banners section

https://www.youtube.com/watch?v=-TqlxFbhRTQ