Guarantor application info & form

Hello everyone

I'm new here so apologies if I'm asking the obvious. I'm processing an application from two sisters, one of whom works full time, and one of whom is a single mum on benefits. Their father has agreed to be a guarantor as they do not quite pass the affordability test. Now, I've never had to get a guarantor myself before, and I'm not quite sure what to look for, or what information I should be obtaining beforehand. Is there a guarantor application form available that I could use? I couldn't find one on the documents section.

Many thanks for any help.