All you have to is earn 15 stars, you can earn up to 3 stars in campaign levels, 1 star in heroic challenges and 1 star in iron challenges. The videos below might help you out (Veteran difficulty):
Campaign Levels:Hammerhold:Sandhawk Hamlet:Sape Oasis:Dunes of Despair:Buccaneer's Den:Nazeru's Gates:Crimson Valley:Snapvine Bridge:Lost Jungle:Ma'qwa Urqu:Temple of Saqra:The Underpass:Beresad's Lair:The Dark Descent:Emberspike Depths:Port Tortuga:Storm Atoll:The Sunken Citadel:Darklight Depths:Bonesburg:Desecrated Grove:Dusk Chateau:Heroic Challenges:Hammerhold:Sandhawk Hamlet:Sape Oasis:Dunes of Despair:Buccaneer's Den:Nazeru's Gates:Crimson Valley:Snapvine Bridge:Lost Jungle:Ma'qwa Urqu:Temple of Saqra:The Underpass:Beresad's Lair:The Dark Descent:Emberspike Depths:Port Tortuga:Storm Atoll:The Sunken Citadel:Darklight Depths:Bonesburg:Desecrated Grove:Dusk Chateau:Iron Challenges:Hammerhold:Sandhawk Hamlet:Sape Oasis:Dunes of Despair:Buccaneer's Den:Nazeru's Gates:Crimson Valley:Snapvine Bridge:Lost Jungle:Ma'qwa Urqu:Temple of Saqra:The Underpass:Beresad's Lair:The Dark Descent:Emberspike Depths:Port Tortuga:Storm Atoll:The Sunken Citadel:Darklight Depths:Bonesburg:Desecrated Grove:Dusk Chateau: