here's what you had to do before fighting Onyxia, the first raid boss in vanilla. nowadays to do a raid you can just press a button and wait a few minutes and
Blackwing Lair Attunement Guide - Blackhand's Command in Classic WoW - Guides - Wowhead
Blackwing Lair Attunement Guide - Blackhand's Command in Classic WoW - Guides - Wowhead