You can approach the problem in several ways. When I designed the piece, I placed a stitch marker at the side "seams". When I was within 2 stitches of the marker, I assessed the pattern of the ribbing and worked the last 2 stitches together so as to maintain the pattern for that piece. Even though it's knit in the round, I still consider the front and back separate pieces. So, if the last three stitches before the marker are k, k, p, I would work a k2tog (decreasing towards the center of the piece), slip the marker, ssk, k.
If the last three stitches are k, p, p, I would have SSP before the marker and p2tog after the marker.
It's fine that, at the stitch marker, the ribbing pattern is broken, in a few rows it will sort out and break again. Just be consistent, always work the last two stitches before the marker in a manner that maintains the pattern for that piece of the garment, and always work your decreases away from the stitch marker and towards the center of the piece.
Does that make sense?