I'm assuming I join the front lft and right sides
Yes. Instead of thinking about knitting the front 5 and back 5 sts together, forget it for now, I'll explain later. You're really going to be overlapping the left 5sts and right front 5 sts. Stop 5 sts before the end of a RS row. Put those 5 sts on another needle, it can be smaller than what you're working on. Then put the needle with the other end of the row next to the 5 sts on the holder, holding them together parallel Knit one st from the front needle, and one from the back needle together (this is the front and back that's referred to, not the front and back of the piece). Look at the video for the 3 needle bindoff
to see how the needles are held and how you knit sts from each of the needles together. You just won't be doing a bind off here, otherwise it's the same.
Now the fronts are joined and you knit to 18". Then to make the seat belt slit, knit to the middle of the front, turn your work and knit flat (purl on WS, knit on RS) on half the sts, from the center front to the center back. Knit for an inch, then take a new end of yarn (or cut the working yarn, leaving a longish tail) and do the same on the other side, from the center back to the other
center front. Then knit all the stitches in the round again.
When you're at 23" k2tog across all the sts for 2 rounds, then bind off.