How hard?
Depends what you want it to do...
Some are VERY VERY simple, like this:
http://www.asp101.com/samples/shopping.asp

How to do it?
Write it from scratch.
Having a program do it for you is a quick solution that gradually deteriorates and becomes harder and harder to customize/maintain.
Coding it yourself means you have full control of how it works, you fully understand how it works, you can customize it as much as you like and you gain a lot more experience...