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)
sewing machine and thread