How to join squares?

Hi everyone! I’m new to this forum and I’m hoping someone out there can help me…I’m making an afghan (Square Windows Throw, Knit It 2005) and am at the point where I can join a few squares to make the first row… The pattern says “Sew squares tog invisibly through back loops of cast-on row…” I’ve only ever sewn up a sweater…That was using mattress stitch, going under the bars between stitches…How do you sew “invisibly through back loops?”

Thanks in advance!

Welcome!

How about this?