Here's what the goal is for this segment.You'll notice that the chair back is longer than the seat. You'll also notice that the two have a curved area with a hole. The goal is to align the two curves and the two holes.
Place the seat and back on the floor and carefully slide them together with the chair back overlapping the seat. Then fold this joint to a not-quite right angle. You'll get something like this:
The holes won't completely align. Once the holes are at least partly aligned, if you have a handy-dandy sturdy screwdriver, you can do this: Insert a strong screwdriver and wiggle the shaft. That will help align the holes.
The final look: The curved edges are aligned as are the holes. In a later step, you'll be able to insert a bolt through the hole in order to attach the chair to its legs.