The Linen Closet located on the second floor of the Spellman House is used as their portal to the Other Realm. Whenever one wishes to go to the Other Realm, they simply go into the closet, close the door, make a left at the towels and follow the signs towards the black hole. When the person(s) are transported, you hear thunder rumbling from the outside of the door and see a bright flash of light shine through the cracks. When people from the Other Realm visit the Linen Closet, there is a doorbell that they ring to alert the Spellmans that they have arrived. The only mortals who have been in there were Jenny Kelly, Harvey Kinkle, Valerie Birkhead, Mr. Kraft and Mrs. Quick.
Oddly enough, during Season Seven, when Roxie and Morgan move in with Sabrina, she doesn't disconnect the portal, making it a potential threat to her mortal friends. At the end of Season Seven, she mentions having it be disconnected after she moves out of the house.