A quick hand basting will hold the lace in place MUCH better than pins ( the pin is marking where the machine stitching began ). Using a small zigzag stitch carefully stitch around the outside edges of the lace piece. 

** It is a good idea to have a piece of interfacing on the inside of the sweater. The interfacing adds very important stability to the stitches 

After completing the stitching flip sweater to inside and CAREFULLY cut away the sweater inside the stitching outline.  Next trim only the interfacing outside of the stitching outline. 

DO NOT cut the lace, however if you do it is easily mended with hand stitching.

This is how the inside of the sweater looks when completed.

And this is the right side of the sweater.