MD5 Hash tag structure

I need to create a md5 hash tag for a payment solution. It is done in C# MVC 4.5 and angularjs/JavaScript.

This is the first time I need to create that form of tagging. Could you recommend some good reading about this?

What is the basic structure of a hash tag? And how do you use the key. How is this encoded and decoded?

What is good practice, and what is bad?