From the FAQ:
When exactly can I use an item? Can I use it in the middle of a movement or other ability? Can I use items outside of my turn?
Items are only used during your turn unless it is used as a reaction to an enemy (as per its wording) or during a granted out-of-turn action if the action matches the item's restrictions. During your turn you can use an item at any time, including during a movement or after you have taken all your actions, with the following exceptions:
- Must meet all restrictions listed on the card
- If an item affects an attack (e.g. adds a bonus, an effect, advantage or disadvantage), it has to be used before an attack modifier is drawn
- If an item grants an action, it cannot be used in the middle of another action