Press N and wait for the cutscene with Nearly Headless Nick to end. Afterwards, follow the ghost , which will lead you to the potions lesson. After exiting the Gryffindor Common Room, head down the Grand Staircase .
At some moment turn right after Nick  and afterwards go through the big door on the left .
You will get to the Entrance Courtyard . Go across the square and follow the ghost into the corridor on the left . Exit through the passage on the right and continue to follow Nick.
Eventually you will reach the Viaduct . You can press Shift in order to get to the other side quicker. At the end of the road, go through the big door and you will enter the Viaduct Entrance . Go into the corridor on the left.
Soon you will reach a staircase , go down using them. Afterwards go pass the crosswise corridor and enter the Potions Classroom  in front of you.
After the conversation with your teacher, a potion tutorial will begin . Just perform the tasks displayed on the right side of the screen. Don't get upset if you have problems making potions and try again. You'll get a hang of it eventually. Here's a little tip if you have problems with pouring liquids into the pot and you spill them onto the table. Try to place the bottles above the pot in such a way that their shadows are at the top edge of the pot. Tilting the bottle in such a position will cause the liquid to hit the exact spot. Remember to add the ingredients with care. If you add too much, smoke will burst out of the pot . In order to get rid of it, put back the item you're currently holding and move the mouse above the smoke.
After you learn how to add the ingredients, you will have to warm up the mixture . To do this, just move the mouse up and down at the bottom of the pot. After completing this task, you will have to prepare some potions. Once you're done with that, exit to the corridor through the wooden door .