fill in sth or fill sth in
to write the necessary information on an official document [e.g. form, questionnaire]
Please fill in the application form and send it back by November 2nd.
to cover a hole in the surface of something and make it smooth by putting a substance in it
Before painting, fill in all the cracks in the plaster.
:)tak zalazłam w: Cambridge Dictionares Online
(from Cambridge International Dictionary of Phrasal Verbs)