Beginer off-page SEO

Hi guys ! I am a total beginer.I have experience in web design but not SEO.I have created 2 money sites in good niches that rank well ( 2nd page ), just with on-page SEO.

What would be your advice on beggig to create backlinks to those money sites?

I am also a bit tight on the buget, and i can't start my personal PBN for the moment.Is there any chance i can do some off-page without money?