To set-up your seaming, lay the pieces flat and line everything up evenly. Try using claw clips to hold everything together to stay on track while you work. You may notice that the vertical piece has more give and can be stretched much longer than the horizontal piece. You can either eyeball it like Julie (using the claw clips makes this strategy much more accurate) assuming the ratio is somewhere around the average of 2 to 3. Or you can count all of your stitches/rows and mathematically figure out the proper ratio.
For the vertical edge, you're using a Mattress/Blind Stitch. For the first pick-up go in between the first and second stitch and find two bars to pick up. If you're eyeballing the seam ratio, continue to alternate picking up one or two bars.
We recommend trying our Tapestry Needles while seaming. They're unique because they have a bent tip which slides easily through stitches, making the pick-up process much quicker.
For the horizontal edge you look for the "v"s going through the points where they come together at the bottom. Continue picking up only one v for every pick-up.
As you go, pull on the yarn to clean up the seam. If you pull too tight, simply loosen to the proper tension by gently pulling on the seam line. Once you're finished, Work in Ends with the Duplicate Stitch, and cut the tail with our yarn snips for the most delightful "snip, snip" sound to complete your project.