It's not much of an ambush, so you can blunder right into it if you want. Eventually (#2), Galmar will stop because he'll suspect an ambush in the next room, and he'll ask you to scout ahead. Inside, your allies will start exploring the Temple, killing any Imperial soldiers that they spot. Galmar and Ralof can't be killed, and if any of the soldiers die, then they'll just get replaced, so you shouldn't have any trouble with the battle. After a brief speech, your allies will charge at the Imperials camped at the entrance to the ruins, and then they'll go inside. You'll have to travel to Korvanjund on your own, but when you get there you'll find Galmar, Ralof (who you might remember from Helgen at the start of the game), and several Stormcloak soldiers waiting for you. Jagged Crown - 'the symbol of the righteousness of our cause' - to the ruins of Korvanjund, and he'll ask you to help him retrieve it. Galmar will tell you that he's tracked down the You'll receive this quest from Galmar Stone-Fist immediately after completing the quest

Skyrim how to keep the jagged crown