print mailing labels Sigep

I need to generate mailing labels and print, so based on the GitHub link project, but I'm having trouble rolling the project in my environment, I made installation with composer, in the manual talks to download the FPDF 1.7 library and set up to auto load before printing, so beginner in PHP that part I did not understand with does, when vo print with this library the error Can not modify header information - headers already sent by someone can help me?