You can find full documentation here.
You're on the right track, but cluster-id should be greater than 0. From the documentation:
> Setting a cluster ID to 0 is equivalent to disabling a cluster. A cluster ID greater than 15 can only be set when the fabric and control link interfaces are connected back-to-back.
Keep in mind that you need to connect the two SRX300 devices on port ge-0/0/1 (for the control link). In the factory default configuration, this interface is already configured and part of a security zone; you need to remove all configuration relating to this interface before the chassis cluster will form.