You need to be a champion with all horde factions. In order to be a champion you need to collect 25 valiant’s seals per faction. You collect these seal’s by doing the dailies. You can collect 5 per day max.
It takes about a month of doing the dailies. You need to be exalted with all the horde factions plus the argent crusade.
It costs I think 100 champion’s seals but I wouldn’t worry about those as you’ll get a bunch just by doing the dailies once you’re champion with a faction.
In 5 days you’ll be champion with that faction, not exalted. For the rep part ya, dungeons with tabard is probably the best way. You will also get rep from doing the dailies, can’t remember how much though. I think it was about 275 per quest.
Also, when you become a champion with one of the factions you unlock more dailies that reward you with a champion’s seal and either 10 gold or a champion’s writ. You can exchange the champion’s writ for 250 rep each.
I wouldn’t worry too much about argent crusade rep, you’ll be exalted by the time you become champion with all the factions just by doing the dailies.
When you become a champion with one of the factions, pay attention to how much rep you get during that process and use that to calculate how much extra you’ll need.
It’s technically 27 days because you have 3 days of Aspirant dailies before you become a Valiant. However the final day of Aspirant you can start your Valiant quests so it’s only 27 instead of 28.
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Also, after the valiant grind is done, you unlock the last hub of dailies and the turn it rewards either extra gold or scrips which can be turned in for bonus rep. So that can help you bring up the things that lag behind.
If you have a faction you have a lot of low rep on, once you get 25 valiant seals, dont turn it in yet. Just keep doing the dailies with that city. You’ll overcap and get extra seals which will count towards the next city, and you get to do extra dailies that target that factions rep.