After exiting the classroom and watching a cutscene, head to the corridor on the right . Cross it and approach the painting  leading to the Grand Staircase.
Call Nick and follow him downstairs . You will once again arrive at the Entrance Hall .
Enter it and approach the big door on the right . Go through it and you will get to the Great Hall, where the Gryffindor Dueling Club is located. Once you're inside, walk onto the platform in the centre  and approach the student in the middle.
After the cutscene, training will begin  and you'll learn how to use the Stupefy and Expelliarmus spells. Afterwards there will be a practice duel. It's not too hard, just dodge the enemy attacks and attack when you see an opening. You can try to knock him over  and repeat the spell as he tries to stand up.
After the fight, you can head to the wooden board  hanging to the left of the door through which you entered. Thanks to it you can fight with four club members. In the end a champion will await you. If you manage to defeat them all, your duel spells will become more powerful. Whether you want to fight them or not, you can exit the room through the big door  that you used to get here.