What best practices should be used when entering data from excel to web form?

Hello all,

I am working on a task that needs to be done following best practices, it is to generate excel calculation results and to fill a web form with the data.

I have automated the task but I dont know what are best practices, sometimes the input is blank and I think i need to have some error handling.

How can i handle an empty input or invalid input to web form?
What are best practices to be followed?

Thanks