Nick here from Shopify.
Can I ask what you mean exactly by offline payment method? Do you mean a retail point of sale (POS) payment gateway?
The best and easiest way to build an SDK payment gateway within Shopify is to fill out the SDK request form here. You can see the help guide Shopify wrote for this here also. Once you fill out the form, someone from the team should be in touch with you within 30 days. If nobody is in touch with you, it likely means your request is not approved.
All the best, Nick
No problem at all @mansi1695, happy to help.
In this instance, I would say the request form is still the best way of moving forward with what you are looking to do. Currently, it would be the only way to get in touch with the team who work with Shopify Partners or Experts about building SDK payment gateways of any kind. The reason for this is because access to the Hosted Payment SDK is limited so if you want to build a payment gateway, then you'll need to be granted access to it first and the request form would be the only way to do that.
I have also moved your post to the Shopify API and SDK board where it is more appropriate and will get better eyes on it.