how get player heads:
replace templete head with your skin in /textures/player_heads/
replace templete head preview with image in /textures/player_heads_inventory/
rename your head in /texts/en_US.lang
go to https://www.uuidgenerator.net/ and generate 2 uuids version 4
open manifest.json and replace first 2 uuids with new that you generated