How to Cut a T Shirt Into a V Neck
To cut a t-shirt into a V-neck, start by using a V-neck shirt you already have as a guide for where to mark the "V" on the t-shirt. After you've traced the "V," use sharp scissors to remove the stitches securing the front of the collar. Next, cut out the V-neck, then cut the detached collar down the middle, stretch it along where you marked the "V," and pin it in place. Finally, sew it down, starting at the top of the collar and placing the stitch about 1/4 inch from the edge of the layers.
Gather your materials. To complete the project, you will need a crew-necked shirt, a ruler or measuring tape (if using a tape measure, you'll also need a separate straight edge), stick pins, a fabric marker, fabric scissors, a seam ripper, thread the same color as your shirt, a sewing machine or sewing needle. Measure the V. An easy way to do this is to use a V-necked shirt that you like as a guide. Fold the shirt vertically in half, making sure that the shoulders line up. Lay it flat on a table.Then use a ruler to measure the distance from the point where the collar meets the shoulder seam to the point of the V. Write down this measurement. If you don't have another v-necked shirt, you'll have to estimate how deep the V should be. In this instance, it's better to start conservatively as you can always make it deeper. You may want to try on the shirt to gauge how deep you want the V. While wearing the shirt, look in the mirror and mark the spot you'd like the point of the V with a pin. Fold your crew-neck shirt vertically. The front of the collar should be on the outside of the fold. Make sure that the neckline, the shoulders and the arms match up exactly. Lay it out on a table, smoothing it down until it is free of wrinkles. Trace the V. Place a ruler in a diagonal line from the the point where the shoulder seam meets the collar down to the middle of the chest. Using the measurements you took in the previous step, mark off the point of the "V" with a fabric marker. Then draw a line between the mark and the point where shoulder seam meets the collar. Flip the shirt over and repeat this step on the other side. Remove the stitches. 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. Smooth the crew-neck shirt out on the table. 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. Cut out the V-neck. 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. Cut the front of the detached crew collar in the center. You'll need to first determine where the center is. To do this, lay the t-shirt flat with the front facing you. Measure the width of the collar, and using your fabric marker place a dot in the middle. This is where you will cut. Stretch each side of the cut collar along the length of your V-neck. Most crew-neck t-shirt collars are ribbed and should give several inches. Pin the rough edge of the collar to the shirt. Stretch one side at a time along the length of the V, pining as you go. Place pins approximately every inch (2.5 cm) to make sure the collar stretches and stays before you sew it.  Do the same thing on the other side. The rough edge of the collar should be paired with the rough edge of the t-shirt, with the edge of the collar facing the outside of the shirt. Sew from the top of the collar down to the bottom of the V. Sew approximately a quarter inch (0.6 cm) from the edge of the two layers. When you sew down the second side of the collar, stop just a little before you get to the V and sew that end to the back of the first side. Finish by pressing the new hem down with an iron. Make sure the thread in your sewing machine matches the color of the shirt. If you don't have a sewing machine, you can also hand sew the sides of the collar to the sides of the V. 
Gather your materials. Measure the V. Fold your crew-neck shirt vertically. Trace the V. Remove the stitches. Smooth the crew-neck shirt out on the table. Cut out the V-neck. Cut the front of the detached crew collar in the center. Stretch each side of the cut collar along the length of your V-neck. Pin the rough edge of the collar to the shirt. Sew from the top of the collar down to the bottom of the V. 