Layout your hexagons to stitch together. Make sure the lengthwise grain is vertical (shown by the blue arrow). The crosswise grain should be horizontal (shown by the purple arrow). The first step will be to sew the long columns together vertically.
Generally you mark the seam lines on your hexagons to make sure 1/4" is left open for nesting seams. After sewing lots of hexagons, it hit me to mark my machine rather than my hexagons. Then I just have to mark it once to sew lots and lots of hexagons.
Set one hexagon aside and mark 1/4" seam lines with a dark pen, making sure the lines are easy to see.
Place the marked hexagon on your machine as if you are beginning to sew a seam. Put the needle down on the black line and make sure the edge is carefully lined up along the 1/4" seam line.
Mark your machine with washi tape along the edge of the fabric. This tape will act as a guide when stitching hexagons together such that marking each hexagon isn't required. I like washi tape because it's thin and not super sticky. It shouldn't leave a residue on your machine. I prefer a solid color that's quite different from the colors of the fabrics I'm sewing. Target has washi tape packs that come in four different colorways.
Repeat the process for the bottom of the hexagon. Line up the edge of the fabric along the 1/4" seam line and place the needle down right on the black line.
Now with your machine marked, you're ready to stitch. Match up two hexagons, ensuring that the grain lines are running in the same directions as shown above. Line up the hexagon with the first piece of washi tape. Stitch a 1/4" seam making sure to backstitch well.
Stitch a 1/4" seam and stop stitching when the fabric is lined up with the second piece of tape. Backstitch well.
The stitched piece should look like this. You want an open 1/4" at each end of the seam.
Continue stitching hexagons together until each row is complete.