Help! Wrap with slits

This is a redheart pattern wr1811 and I'm confused on my center section in making the slits. I'm on the center sectionwhere I'm to join yarn in 17th st on row 21. Do I join before 17 or after 17? Then I'm to do 2 hdc which isn"t following the previous pattern. I've included a link to the pattern and any help for this new crocheter would be greatly appreciated!!