You must have your business already registered with BSNL DLT to register your templates.
BSNL DLT Template Registration Steps
First Login to BSNL DLT Portal as Enterprise here https://www.ucc-bsnl.co.in/
Then click on Templates>> Content Templates option from right menu.
After that click on add option from right corner. Then you will get add new content template screen.
Choose template type first Promotional / Transactional / Service. Choose Service >> Implicit category for OTP and service SMS after registration.
Then select already approved header from which this sms will be sent.
Then type template name for your reference.
Then add sms content in template message box.
In message where SMS data gets changed everytime for every user there add insert variable from above.
Sample : Dear {#var#}, Your OTP is {#var#}, Team ABC Software.
Then click on save.
After that you will be redirected to template list where you can see approved / pending templates. Once template approved you can use it any time.