How to create a Paypal form?

I have tried some of the demos but I can not seem to edit anything. I just want a simple form for people to order 1 t-shirt at different sizes for 20. What is an easy way to do this.

To learn on how create your order form, please do follow the guides below:

https://www.jotform.com/help/47-Order-Form-Basics 

https://www.jotform.com/help/49-Order-Form-Types 

https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form 

Please note that you can only add one payment integration on a form, you can edit it by simply clicking the payment field and click the wizard/wand icon.

Here's a quick screencast on how to add a payment integration, create a product and re-open the wizard:

I hope that helps.