How do I set payment terms where I take payment in 30%-40%-30% as the project progress ?

How do I set payment terms where I take payment in 30%-40%-30% as the project progress ?

Hi,

I make a Invoice a project, and I take 30% in first payment, then 40% as the project progress and then 30%.

This % may vary from project to project, and the dates also vary in between the payment times.

So, how do I create Invoice for that ?