When you sew a half hexagon together you end up with a piece that has two parallel edges and two edges at an angle. If you lay one of them down with the longest edge facing you, you can think of it as an "M" shape. If you turn it half way around you can think of it as a "W" shape.
You sew a row of alternating M's and W's into a long strip.
Lay it out so you can place the matching half hexagons in the right order next to it, so now it looks like a strip of full hexagons.
M W M W M W
W M W M W M
Pin and sew the second strip of half hexagons alternating W's and M's as you did the first strip.
You'll now have two long strips with long parallel edges that you can join together along the long edge, and hopefully your hexagon halves will be aligned in the right order so you end up with a full row of hexagons.