Where to start sewing? Most of the time I start at the narrow end and sew to the wide end. It's so easy to "drift away" from the cut edge at the narrow tips, making the seams less than 1/4". When someone says their circle poofs up in the center, it is usually because their seams are a tad bit scant at the narrow portion. The seams must be the same width from narrow tip to wide end for the circle to lay flat.
Often the pattern will suggest specific units to sew together, so obviously follow the order the pattern suggests. But if you're on your own, I suggest you sew pairs, then put the pairs together. Some recommend making quarters, then halves, then finishing the circle and pressing as you go. I don't do it that way! I put the whole circle together, then press. More about pressing next visit!
Look for Ten Degree Tuesdays blog for more tips and tricks on getting creative with your Ten Degree Wedges!