From your port page, click the 4th bottom button from the left. The button is blue and should say "好友". This takes you to your friend list.
Then click on one of your friend's profile, and a small window should pop up. Click the blue button called "拜访". This will take you to your friend's profile page.
From here, click the bottom left button on his profiles below his medals. The button says "挑战" and should be a picture of a plane, a target scope and an arrow. This takes you to his current fleet comp.
Click the orange button called "挑战" at the bottom right to fight their fleet. You will use your 1st fleet just like in normal drills.