Alternative flow in sequence diagram


I’m working with UML for a few weeks now. I created a very simple sequence diagram (a basic flow).

I don’t know how to model my alternative flow. How far do I go with this?
For my sales screen there are several values which need to be validated. Like: is this the right customerID, is this address correct, is this contact person correct, etc.

Can any body help me to create an alternative flow with all kinds of validations, please?