Slide open the spring-loaded clamp & adjust the little contact legs so they align with the terminals on your battery. Push your battery against the teminals. Release the clamp to hold your battery in place & plug the unit into your power source (AC, DC car/solar, USB computer/solar)
Adjust the battery contacts if necessary. The charging lights will show the charging status.

How big a battery will it hold?

Must be a 3.6/3.7v or 7.2/7.4v lithium battery pack. This covers cell phones & most digital cameras.

The battery can be as long as it wants. But the battery terminals must be on one end of the battery. This charger is able to accommodate slots, tabs, or pin connectors.
Max battery width: 4.5cm/1.75"
The spacing between the terminals must be no closer than 1/8" (3mm), and no wider than 1.75" (45mm). Get out your ruler!

Is it safe for my batteries?

Yes. The charger automatically detects the correct polarity of your battery, so you don't need to worry about +/- terminals. It also charges at the appropriate voltage for your lithium battery, which it auto-detects. A series of 4 LEDs let you know the charging status.