First, you need to become an Annual Member or buy a 24-Hour Pass to be able to use Divvy. Then, find available bikes on the System Map.
If you're a member, you can insert your bike key into a dock to unlock a bike; or if you don't have your key with you, you can use Transit App to get a mobile ride code. If you purchase a 24-Hour Pass from a kiosk, you'll get a ride code, which you'll type into the keypad on the dock. Once you've successfully inserted your member bike key or entered a ride code, the light on the dock will turn green. This means the bike is unlocked. To remove it from the dock, pull the bike out by the handlebars or lift the bike by the seat to release it.
To ride again while your pass is still active, just request a new ride code using the same payment card (don't worry, you won't be charged again).