How did the children get into Narnia to begin with?

They first got into Narnia through the magical rings. They used the ring to the wood between the worlds, then jumped a pool which could lead them to Narnia.

How do they leave Narnia and get home?

They used the ring to leave Narnia and came back to the wood between the worlds. And Aslan also went along with them. Aslan sent them to their world without using the rings and this process was proceeding under that they even unawared of it.

What happened to the Apple which Aslan gave to Digory?

The brightness of the Apple made all sorts of colored things in the bedroom that seemed to have scarcely any color at all.  It threw strange lights on the ceiling. Nothing else was worth looking at. The smell of the Apple of Youth was as if there was a window in the room that opened on Heaven. 

What warning and command did Aslan give to the children before He sent them back to their own world?

The warning:  Perhaps some wicked one in children’s world would find this secret as evil as the Deplorable Word and used this way to destroy all living things. Great nations in children’s world will be ruled by tyrants who care no more for joy and justice and mercy. Aslan informed children that let their world beware.

The command was that Aslan required children to bury all magic rings to make sure no one to used them again