You'd basically just do the same thing for entering, but in reverse. You also need to fade out and fade in to make it look smooth.
Change screen color tone to black
Transfer player to the door (on top of it, if that's what you want)
Change the door graphic to the "open" state
Use screen color tone to change the screen back to normal
Turn "Through" on for the player
Move the player down
Turn "Through" off for the player
(If you use one event, check the event ID of the door to move it and stuff. Move commands work on the map the player is currently on (I believe, haven't played around with it for a while). So if the door is 002 on the outside map, move the 002 event ID, even if 002 is a bed or something on the inside.)
The main difference is that you need to add the fade out and fade in parts manually so that you can control the door graphic before the screen displays. You could also keep the door graphic the same, but make the player invisible, and then open the door, make the player visible, then turn through on, step down, and turn through off.
I can go into more detail if you need it, but that's the general idea.
