I've recently begun trying to understand and implement nonweapon proficiencies as written in the AD&D 2e Player's Handbook. Most aspects of the system seem to come across clearly enough; my understanding is that when undertaking a task for which one of their nonweapon proficiencies applies, a character makes an ability check. Ability checks in 2e succeed if the result is equal to or lower than the relevant ability. (For instance, a character with STR 16 making a STR check succeeds on a 16 or lower.)
The confusion comes in when you start adding modifiers. For example, Fire-Building requires a Wisdom check with a -1 modifier. Given that fire-building should be a reasonably easy task, I would assume that the "-1" here is a bonus — in other words, it applies to the roll to make the task easier. However, Engineering requires an Intelligence check at a -3 modifier! Engineering is certainly much more difficult than Firemaking, so I would expect the modifier to be worse for the character attempting the task. Thus, I must assume that modifiers to ability checks affect the target number, rather than the die roll.
This seems to be the logical conclusion (particularly as the blacksmithing example in the paragraphs following the table makes note of a "-3 penalty") but I felt it necessary nonetheless to seek confirmation:
How exactly are modifiers applied to ability checks in AD&D 2e, particularly in the context of proficiency checks?