How can i write a letter of reinstatement for my old employees to re-accept me ?

hi all, i work in the banking sector and have switched banks almost 2 years ago due not getting the grade i deserved. since then i switched to a different bank where i have gotten a higher position and would love to go back to where i started. i was told that i have to write a reinstatement letter and would love it if you guys can help me !