1. The 'Nop.Plugin.Payments.Paytm' directory contains source code.
2. Copy it into \Plugins directory on your server.
3. Build this.
4. Go to admin > configuration > plugins > local plugins
5. Install paytm module.
6. Go to admin > configuration > Payment > Payment method > paytm
7. Fill in the details here.

This plugin is updated to Nop version 4.2 from official Paytm Plugin for NopCommerce