How to reminded a Prof of a promised recommendation letter (2 years ago)
Write a letter, send an email.

Say, 


  Hello, I am user3222184, and when I was an undergraduate, I
  volunteered at your research lab in 2013. I presented xyz work to you.
  
  I am pursuing graduate studies and would be honored if you can write
  recommendation letter for me.
  
  Thanking you, user3222184