who is an ATG

what does one have to do to become one

who is an ATG

what does one have to do to become one
Someone who is a notch above "very good."

What determines an ATG is up to personal scrutiny as are the requirements. I believe it's being in lets say the top 150 boxers P4P or Top 15 in an original weight class. (ie moving JWW to WW etc.)

All time great

Somebody who beat the best during his era and showed skill, heart, consistency and longevity and dominance over a weight division.

Wins over Hall of Famers and other fighters considered to be ATG would increase somebody's ATG standing.

You should also try to consider what the boxer achieved in his prime and how they overcame adversity (e.g. avenging defeats).