THAC0 is tolerable in precisely one situation: the GM tells you your opponent's AC. Then, you roll your d20, add your modifiers and the AC you've been given, and if you match your THAC0 or better, then you've hit.
Example 1: THAC0 of 15, AC 3, +2 to hit. Roll a d20, get 13. So, 13 (d20 roll) +3...