SenderID Registration is an important part of the DLT registration process with the operator.
You will need to have done the initial registration across operators to do the SenderID registration. Not sure on how to do the Entity Registration? Read about it - Here.
Once the Entity is registered and you have successfully received the EntityID, you can go ahead with the registration of your senderIDs across any of the operators as mentioned below:
SenderID approval or rejection by the operator usually takes around 24-48 hours.