First follow the guides at:
After you have your full subnet code ready, according to all of the predefined requirements. Proceed with registering a new subnet storage.
You can register a new subnet by proceeding with the following command:
comx subnet register <key-name> <subnet-name> [--metadata]
Note that you will be asked to pay a subnet burn fee, these tokens will be permanently lost. To determine the cost beforehand, you can query:
comx subnet params
and look for the subnet_registration_cost field.
After you registered your subnet, you are still not eligible for an emission distribution. For emissions to start your subnet there will need to be enough consensus weight set on it by rootnet validators.