This guide will go discuss and explains my approach to Mage PvP BIS Gearing in Phase 1 of Classic Wow.
Progressive Itemization And You
Mages are one of the classes that suffers the most from progressive itemization. Early Gear from dungeons and raids prior to 1.10 was mostly stat stick kind of gear that didn't have much in the way of spell damage. Melee, on the other hand, have a lot more gear to take from because their base stats (agility and strength) provide damage, while intellect does not- meaning that our PreBIS / BIS for Phase 1 is in a much better state in Phase 1 of Classic than it would be with Progressive Itemization.
Early PvP Stat Priority
Stamina = 1.2 Spell Power
Intellect = .4 Spell Power
1% Crit = 10 Spell Power
Early Stat Breakpoints
This is a topic a lot of people get wrong so let me explain it in detail. In PvP for a target of EQUAL level you have a base 96% chance to hit on your spells. You want 3% hit because you can only increase your chance to hit up to 99%. In Phase 1 this means that you want 1% hit from your belt (Ban'thok Sash) or Neck (Star of Mysteria). Having both does not matter because you will still need 1 point in both Elemental Precision (2% Hit for Fire and Frost spells) and Arcane Focus (2% Hit for Arcane Spells) to reach 3%. This is also why you want to avoid having 3% hit from your boots or helmet, or trinkets, because these items are the ones you swap the most (Engineering Helms, Mithril Spurs, Carrot on a Trick, Rocket Boots etc.)
HP and Mana
While the above can generally be followed, int and stam will be worth more while you are below ~6k Mana and ~3k HP- this mostly applies to PreRaid BIS where you can go with heavy spell damage pieces like Spirit of Aquementas while your base stats are still fairly low. Prioritizing those breakpoints early on help ensure you don't get bursted down too quickly or run out of mana while preserving most of your damage early on because it largely comes from base damage and talents.