Altering the neckline of a garment alters the appearance in a drastic way. This affects not only the overall look of the garment but how well it displays your features. Most alterations take away from the shirt instead of adding onto it, but both are possible with enough planning and sewing knowledge. Adding onto the shirt is particularly difficult because you still want it to look like a cohesive garment instead of a shirt with a poorly added collar. Practice your alteration technique on some shirts you aren't fond of before you attempt to alter your favorite garment.

Measure the size of the neck on your sweater. Cut a piece of elastic material close to the original material of your sweater.

Lay the square of fabric on a flat surface. Cut it to twice the length of your throat. Cut the fabric in half.

Fold over a 1/4 inch long piece of the fabric on both the top and bottom (the sides that are twice as tall as your throat). Sew it in place to form a seam.

Turn your sweater inside out and lay it on a flat surface. Place one piece of your fabric along the front neck line on your sweater. Pin the fabric in place and position the second piece of fabric in a similar fashion on the back neckline.

Sew the new pieces of fabric into place on the existing neckline. Sew the edges of both pieces together as well, leaving one long tube of fabric sticking up above your existing neckline.

Put on the altered sweater. The neck is very long, so fold it over once to make it into a neck-hugging turtle neck. The folded over fabric obscures the original neckline, making the turtle neck seem more natural and suiting.

Turn your shirt inside out. Draw a new neckline onto the front and the back of the shirt with a chalk pencil that is 1-inch above where you want the neckline to end. The pencil marks clean off easily in the washing machine.

Rip out the seam on the shirt that runs along the neck. Remove the excess piece of fabric from around the collar of the neck. Separate the seams on the shoulders as well to make the hole for the neck larger, ending the seam ripping 1-inch below the chalk line on either side.

Cut the fabric on the sweater along the chalk line. Remove the excess fabric once you finish. Measure the length of the sweater neck along the new front and back line.

Cut a length of slightly elastic fabric to the size of the neckline on both the front and the back of the sweater. The fabric lines should be at 2 1/2 inches wide. The slight elasticity will keep the shirt from stretching out and warping easily.

Fold the ends of the fabric strips over to make a seam on either side. The fold should be 1/4 of an inch on either side. Sew the lines in place to keep the fabric from unraveling.

Place the lines of fabric along the front and back lines of the shirt’s new collar. Position them so their seams touch the shirt instead of facing outward. Place it so it lines up with the top edge of the shirt on the front and the back of the neckline you cut.

Fold 1 inch of the shirts neck back towards you. This makes it so 1 inch of the new fabric piece is visible on the front of the sweater while 1 inch is visible on the back of the sweater. Sew along the bottom edge of the fabric line, securing it in place as a new, smooth neckline for the sweater.

Sew the shoulder seams together again so you have a strong, cohesive neckline. Turn the shirt right side out. The neckline should be adjusted as low as you want it with a neat string of fabric going across the collar.