just a small warning: I'm not sure what stag's bodily features are right so I apologise if anything is wrong.

At exactly 7pm the group arrived into the room of requirement.
Inside were Professor McGonagall, Sirius Black and Remus Lupin.
Harry, Ron and Hermione walked into the room and sat down on the cushions on the floor.
"Welcome, I hope you all are ready, because we're going to try the hard stuff straight away." McGonagall said.
"Professor, I brought the book, will it be any use?" Harry said.
"Of course, if I remember correctly, someone else used the book and it helped them become excellent animagi." She said looking at Sirius.
"First of all, you need to picture an animal in your head. Pick the animal in your patronuses. So that would be a stag for you," she told Harry.
"Stare at your hand for a while, and then imagine it the hand of a stag. Or maybe the leg of a stag, give it a go."

Harry stared at his hand, but then decided to change his leg instead. Focusing on his leg, he blinked and tried to see the leg of a stag. Nothing happened.
He was going to try again but Lupin stopped him.
"Harry, you remember how to create a patronus. Instead of picturing a happy thought in your head, try to think of the animal. The way it walks and looks. Go on, give it another go," Lupin said.

He stared at his leg and then blinked, picturing the stag in his head. When he opened his eyes he realised that his legs had changed. He had hoofs instead of feet.
"Whoa, how did that happen?" Harry said looking at his feet.
"Now it's the tricky bit, think of your own foot. Try to turn your feet back to usual." Sirius said.

Harry closed his eyes trying to think of how he looked before. When he opened his eyes, it all stayed the same.
"Why isn't it changing?" He said trying not to believe he was stuck with these hooves.
"Let's leave it at that for now," Professor McGonagall said with a smile on her face. "We'll make you back to normal in the next session." She looked at Sirius, nodded, and then walked out of the room leaving Harry without his feet.
"Finite Incantaum," Lupin said pointing his wand at Harry. His feet returned and the hooves disappeared. "Ron, Hermione, why don't you have a go?" Lupin said. "Try to think of an animal."

Minutes later, Hermione has changed her hands into tiny otter hands. It took Ron a little bit longer to change into a Jack Russell terrier but they all succeeded. At the end of the session, Lupin gave them a block of chocolate and left the room.
Just before Sirius left, he said, "Keep practising in here, we'll arrange the date of another lesson soon."