I’m not sure if I should purl the next row after I did the ‘3rd inc row’ before I do the ‘1st dec row’…?
You’ve got it marked in pink - it says “stockinette stitch for 13 rows”. So, yes, purl the next row, but also conintue on for 13 total rows of purl row, knit row, purl row, etc. - for 13 rows.