The British sailed down the St. Laurence River in small boats.
The British spoke French to make the guard believe that they were French allies. They ended up tricking him into leaving his post.
They climbed up the side of the mountain.
They arranged themselves into rows and waited for the French to notice. The French were not used to this attack and all needed to reload at the same time, the British held fire until that happened.
The whole battle only lasted less than an hour. Both the British and the French lost their generals. The French ended up surrendering to the British, meaning that the British now had control over Quebec.