- Get your class' to-hit number from the appropriate table based on your level (say 9+ for a 2nd-level fighter).
- Subtract all of your bonuses from strength, magic items, dex, and so forth. This gets you for example 7+ for our 2nd-level fighter with a 16 Strength.
- When you go to attack, roll the d20 and subtract your modified to-hit number. Let's say our fighter rolled a 13; he reports to the DM in Traveller parlance that he got effect 6 on his attack roll, or "hit by 6", or something of that sort.
- The DM compares that margin of success to the monster's AC. If it's greater than or equal to the AC, it's a hit. Else, it's a miss.
But yeah, we had quite a time before we switched from subtracting AC from the attack to using it as a target number. I could see where the complaints were coming from, and should maybe send this procedure to Autarch as a "Hey, if you do it this way it's really fast and convenient" type thing.