T-shirt material is so tightly knit that it tends not to unravel, which makes it ideal for quick redesigns that require no hemming, or even no sewing at all. Since T-shirts usually are made mainly of cotton, the material also takes paint, permanent marker or dye well. Add the paint, dye or drawings after cutting the T-shirt, however, to ensure that you won’t cut off or through the design.
Lay your T-shirt flat on your workspace. Place a tank top on top of it, to use as a guide. Cut the front of the T-shirt so its neckline is similar to the neckline of the tank top. Decide where the side straps of the halter should go — a set of straps just above the waist and another just below will usually keep the halter top secure against your body. Cut the front of the T-shirt from the shoulders down the length of the sides of the tank top, but leave strips of fabric on the T-shirt where the halter’s back straps will be. Flip the T-shirt over, and cut around the straps along the neck and back. Tie the halter top to yourself, and cut off the straps’ excess fabric.
Lay a plain T-shirt flat, and place a tank top on top of it. Cut the T-shirt’s neckline and cut off the T-shirt’s sleeves, using the tank top as a guide. Use a permanent marker to draw a pattern of straight lines along the T-shirt’s newly cut neckline or near its hem. Tie-dye the T-shirt or paint it using fabric paint.
Find the areas of the back of the T-shirt where the shirt will rest over your hips when worn. On each side of the hip area of the back of the T-shirt, cut three horizontal slits – one on top of the next. Starting from the right shoulder of the back of the T-shirt, cut slits that angle slightly downward, until you reach the hip slits. Stretch and cut the slits to make them bigger and to shape them – when cutting into the slits, start at the very top or bottom of the slit; don’t start in the middle.
Cut an equilateral triangle with 3-inch-long sides out of poster board. Cut off the top point of the triangle, so it’s a short, flat edge. Place the triangle on the shirt, where the neckline meets the shoulder, with the short flat edge pointing toward the neckline. Trace around the triangle using a washable pen or marker. Slide the triangle further down the neckline, towards the opposite shoulder. Trace it again. Repeat until the entire neckline is ringed with the triangle pattern. Cut along the pen or marker marks.
References and ResourcesWhattheCraft.com: No-Sew Tie Back Halter Tutorial
Annie Spandex: How to Make Tank Tops Out of T-Shirts (Without Touching a Needle)