Sewing pickle dish units together is a slow process with many curved seams to deal with. All I can say to start with is, "Pins, pins and more pins". And, well I guess there is much more to say, so keep reading.
You need to mark all the seam lines and ignore the outside edges. If there is a foundation paper or dissolving product on the pickles, it's easy to just draw on the outside edges. I DO leave the foundation product in for this step. However, if you're using paper, it is much stiffer and you might be better off removing the paper after you mark the seam lines.
Lay out the pieces the way they are going to be sewn together.
Sew the center melon to one side of the pickle dish unit. Pin the ends and the middle, making sure to match up the SEAM LINES (and ignore the edges!). Pin every 3/4 inch or so and make sure to take a tiny bite with the pin. I find that it helps to hold the pieces curving into each other and then let the piece curve upwards while it is in the machine. It's much too hard to try to wrestle them flat into the machine. Sew slowly and only remove the pins when you get close to them with the needle. Press towards the melon. Trim any seam allowances to 1/4".
Sew the two end pieces to the other pickle dish unit. These are also curved, so pin at both ends and put one pin in the center. Use the same technique of curving the pieces into one another as above. Press towards the end pieces. Trim any seam allowances to 1/4".
Pin the two pieces together, using the same curved method above. Press towards the melon and towards the first unit, so that this seam is all pressed in the same direction. Trim any seam allowances to 1/4".
Finally, give the pieces a good pressing from the front. Use a little steam and press with an up and down motion.
I hope this helps you sew pickle dish units together.