By "items" I assume you are referring to Non-Items of Power that are Aspects like, My Father's Old Six-Gun, That Old Bomber Jacket, This is MY Rifle, etc. But Any Item including Items of Power work the same way.
You Invoke it in 3 ways:
1) Invoke for a +2 if the Item aids you in some way. "I use My Father's Old Six-Gun for X"
2) Invoke for effect, you have the item for the scene. Using it in this fashion means that the item is always on your person, it's a part of you as a character.
3) (Similiar to 2) Invoke to make a Declaration, like "My old man was in SF so I'm going so say this Bomber Jacket has Flak built into it" This kind of thing might push things, but that's what you'd do with it. A better example of this would be Harry using his Aspect, My Mother's Silver Pentacle as a Declaration to say he has Inherited Silver. THough this use is more obvious and you wouldn't need it for a Declaration since "Silver" is in the damn Aspect name.
Compels work the same way.
1) Compel for when it hinders you in some way. Such as:
2) Compel to say, "It's always on your person". Good in a Fight, but bad when you are in an Airport full of Cops. Depending on where you are this could mean you get escorted from the premises or Security will draw their guns on you. In the Case of it being taken away from you, this will be a license to print Fate Points if your character hinges on it's presence.
3) Something plot-related to the Item. The Family Cowboy Hat could be compelled to have an Ancient Treasure Map in it.