Upon first entering you'll find that you're now trapped in the Crypt. You'll be in a room containing a bunch of pictures on the wall, feel free to look at them for some background on who Gorfival is/was.
Head out of this room to the south and follow the passage around until you enter a room with a Ghast sitting on a throne. You can either fight it, or answer it's riddle. Once the Ghast is gone look at the section of wall behind where the Ghast was sitting. Enter the secret room and pull the lever.
Exit the throne room to the west and continue following the passage to the temple, which contains a lot of Skeletal Warriors. Kill them and then continue north along the passage into a room containing a bunch of coffins and some Skeletal Warriors and some Zombies. The last coffin to the south contains a Dexterity Charm.
Continue south until you find yourself back in the picture gallery. Check the northeast corner of the room and a small alcove should have opened up containing a lever. Pull this lever and then go back to the room with all the coffins.
In the coffin room there should now be a passage leading off the southeast corner of the room, with a couple Ghasts. Kill the Ghasts and pull the lever in this room. This opens up the exit in the Picture Gallery.
Go back to the throne room now, and a little bit east along the passage you should see a cracked section of wall.Use Move Mountains or a Stoneshatter Crystal to get through. The small room to the northeast of where you just broke the wall contains a Ruby Skeleton and a Zombie guarding a suit of Iron Plate Mail.
Head back to where you broke down the wall and go straight north from there, through the wall. This next room contains Gorvifal and some other undead. Kill them all, checking the ground for any items they might have dropped. Be aware that most of the things in this room are cursed. Check the chest against the southern wall for a pair of Humanskin Gloves, and then check the northern wall of the small passage leading east for a secret room. This room contains a tome which teaches Divine Retribution Lv.3.
Once you're done here go back to the Picture Gallery and up the passage which has opened to the west. This takes you back outside.