My first Dresden was a huge one. What I learned-
1. Use a start and stop fabric to keep your stitching smooth from one end to the other.
2. If it does not lay flat, figure out where the adjustment is needed, and add a narrow seam to make the “volcano” problem disappear. Sew from the wide end to the narrow end, so you are not stretching your blades.
3. Pin, pin, pin, pin! If your ends are not matching up, as long as they are just a little different, they can be trimmed even when you are finished with the plate. If they are way off, you are stretching too much as you feed the blades into the needle. Take the time to carefully remove the seam and try again. Starching well before beginning to cut the blades does make things easier.
Good luck.