|Some Other House Rules|
When magic items are created, the creator of the item melds the elements contained in the item to the energies coursing through the universe. This involves magical skill and the force of will by the creator. Generally, magic items are attuned to the creator, unless the creator specifically creates a magic item otherwise. When someone other than the creator seeks to use the magic item, the prospective user must first be attuned to item through a short simple ritual that also involves the force of personality (Charisma).
Provisions of House Rule
All magic items require attunement. The two general exceptions are alchemy items (potions, powders, oils, etc) and scrolls. Many gnome devices, clockwork or otherwise, do not require attunement (e.g. finder's lens). Some other items may be created with instant attunement, but these items are rare.
There is no limit to the number of items to which a character can be attuned.
Some items, particularly intelligent items, may have the power to unattune other items in the character's possession. Once such an item is unattuned by another item, the character can no longer attune to it.
Some items may only be attuned by a particular class or race. Or there may be limitations placed on its use when a particular class is using it.
Attunement to one item can be done during one short or one long rest. A few items may be created with the feature that it instantly attunes on contact or some other specific predetermined condition, as noted above.
Most items require a die roll for attunement. The Difficulty Class for low power common items is 10 (easy). Characters add their Charisma modifier, plus their proficiency bonus, to the die roll. If the die roll succeeds, the character is attuned to the item. If the die roll fails, then the attunement fails...and, more importantly, the character can never attune to that item (short of a wish spell or divine intervention).
Characters may opt to attempt an "instant attunement" as a free action. The die roll is done at disadvantage. If the item is otherwise unattuned, then the DC is 12, with a critical failure on a roll of 1. If the item is attuned to another living (or undead) creature, then the DC is 15, with a critical failure on a roll of 1 or 2.
Limitations on Magic Item Use
We will follow the AD&D 1e rules regarding limitations on number of items used at any one time: 2 rings, 1 set of armor, 1 magic cloak, 1 pair of boots, 1 pair of gloves, etc. This is not part of the attunement rule per se, but it is related to it as it applies to magic item use (thanks to Peter D for the reminder on this set of AD&D 1e guidelines).
Implementation of House Rule
We will assume that the player characters have successfully attuned to all of the items that they currently possess.
We won't worry about the current bags of holding and portable holes for now. I don't want to waste precious game session time on figuring this out. We will just hand wave it for now.
I can think of two items, the Chalice of Doluth and the Lens of Melnar, that we need to identify an owner for attunement. Attunement will be automatic, once the owner is selected for each item.