1. Fill in the applicant's name, position applied for, and department. A guarantor form is a document that certifies a guarantor's decision to assume liability if a particular individual does not fulfill the terms of an agreement.How do I fill this out? To fill out this Guarantor's Undertaking Form, gather all necessary documents and information beforehand. A guarantor's form should include a space to fill in the home address, work address, phone number, and email address. A guarantor form is a document that allows a person with a financial responsibility to become a guarantor of another person, who needs a loan.