Follow the steps in How do I screen the forms I receive?

Then, in the pop-up, choose the decision you want to record:

1. Email applicant to acknowledge receipt of the application form, and to set out the next steps in your volunteer recruitment process (a 'holding' email)
2. Screened form, progress application, email applicant
3. Screened form, reject application, email applicant

See also, Can I take any other notes?