Summarize:

Unfold the shirt, turn it inside-out and lay it on the table. Make sure that the front side is facing you. Then use a seam ripper to remove the stitches securing the front side of the collar to the shirt. If you don't have a seam ripper, you can use a sharp pair of scissors to carefully cut it away. Stop at the shoulder seams. Unless you don't plan to re-attach it to your new neck-line, leave the collar attached to the back of the shirt. Make sure the connected collar is folded back, away from where you will cut. Doing so ensures the smoothest and straightest cuts and helps you avoid mistakes. Starting on one side of the V, use a pair of sharp scissors and cut along the marked line. Stop when you reach the bottom. Repeat this process on the other side. Be careful to only cut through the front side of the shirt. If you didn't plan on creating a hemmed collar, your new shirt is done.
Remove the stitches. Smooth the crew-neck shirt out on the table. Cut out the V-neck.