How do the % XP Boost items from the battlepacks work?

Press escape ingame and on the bottom right there should be a bit to select which XP boost you wish to activate.

You will then get 1 hour of XP boost. That hour only ticks down while playing. If you activate another one it will overwrite the current one so make sure you use the full hour before activating a new one.

On the Xbox, press Start to bring up the Respawn / Quit / etc menu. In the lower right will be XP boosts you can choose from.

The XP boost items don't stack with themselves; you can only use one at a time. They do however stack multiplicatively with double-XP weekends. Ie if you use a 200% boost, you get 3x experience, so that 100 point kill is worth 300 points. But on double XP weekends with a 200% boost you get 6x experience, or 600 points a kill. Use big boosts during double XP weekends.