I made my insets by knitting, however if you don't like to knit; this could easily be done with lace or any interesting fabric.

Step 1: Gather Materials

Any shirt that you don't mind cutting

Your inset supplies: you could use any lace or decorative fabric. For knitting my pattern you'll need:
less than one skein any medium weight yarn (I used two strands together for decorative effect; you could easily use just one)
knitting needles size 8 and 10 (I used circular because they were on hand-you can use reg needles) (you can also use just one size needle- as explained in pattern)

scissors

sewing machine and thread

Step 2: Measure & Fit Inset Panels

There won't be a 'set' measurement; it all depends on the size of your shirt & how large you want your insets to be. For the shape; you need roughly a half circle; you can see the final shape of my knit pieces at the end of the pattern file.
I just kind of eye-balled it and from there, wrote up my knitting pattern.
Now either cut from your second material (lace, eyelet, any interesting fabric) or follow knitting pattern to create insets.

*this stitch pattern was created from a shawl pattern originally featured on Knitty:
http://www.knitty.com/ISSUEwinter08/PATTmaja.php

knitting pattern.pdf
Download

Step 3: Pin, Cut, Pin....& Sew!

Lay out and pin down your insets on your shirt & mark where you need to cut. Don't forget the seam allowance!
I cut the shoulder seam out, however you may need to adjust, depending on the style of your shirt. Fold the cut edges under, for a cleaner appearance and pin the inset in place.
Now you can sew the insets into place.

and you're done!

Enjoy your fancy new shirt ;)