create new device group: which device will become standby?

Device Management > Traffic Groups > Your-Traffic-Group > Default Device

A default device is the device on which you want a traffic group to run if possible. You actively specify a default device for a traffic group when you create the traffic group. When a traffic group floats due to failover, the BIG-IP system selects the default device as the target failover device whenever that device has the same or more availability as other group members to process traffic. For example, suppose that during traffic group creation you designated Device B to be the default device. If failover occurs and Device B and Device C have the same availability, the traffic group will fail over to Device B. The default device designation is a user-modifiable property of a traffic group.

For additional security you may disable the uplink network interface on the node you wish to become standby. Don't forget to re-enable it once "Traffic groups" and "Device Groups" sections are configured.