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Different straps will require different amounts of fabric, so before buying your fabric you will want to figure out what kind of straps you want to sew on.   The most common strap type is a straight back strap, where the straps are attached to the front and go straight over your shoulders, where they attach to the back of your dress. Another strap type is a cross back strap, where the straps start in the front on one side and then cross in the back and are attached to the other side of the dress in the back. The third basic type of strap is the halter strap, which attach at the front of your dress, go up your shoulders and tie behind your neck. Once you decide what straps you want, you will need to put on your dress to get accurate measurements for your straps. You also need a flexible measuring tape to take the measurements for your straps. The measuring tape will mimic the straps of your dress so you can see how much fabric you will need. Wearing your dress, pin one end of the measuring tape where you would want your strap to start in the front of your dress. Both straps will be the same length, so it doesn’t matter which side you are measuring from.  Make sure you are using the beginning of the measuring tape, the end of the tape that starts at zero. This will make measuring a lot easier. With the end of the measuring tape pinned in the front, put the other end of the tape over your shoulder. Reach back with your arm and pin or hold the measuring tape to where you would want the strap to attach to the back.   If you chose a straight backstrap, the measuring tape should be swung over the same shoulder. If you chose a cross back strap, the measuring tape should go over your shoulder and go diagonally across your back. If you chose a halter neck, the measuring tape should go up your front to the back of your neck. Go to a mirror, still with the measuring tape pinned in the back or held in place with your hand. With your back to the mirror, look over your shoulder. You should be able to see where the measuring tape in the back is being held in place. Take note of the number in inches where the strap should end. You will want to make the length of fabric you purchase a little more than the actual measurement of the measuring tape. This gives you some leeway when you are making the straps.   For straight back and crossback straps, add four inches to the number. For instance, if your strap was 16 inches (40.6 cm), add four inches onto this so the final measurement is 20 inches. For halter straps, add about 12 inches (30 cm) on to this number. You will need enough fabric to be able to make a tie in the back, so you will need more excess fabric.
Decide what kind of straps you want. Put on your dress. Pin one end of the measuring tape at the neckline of your dress. Swing the measuring tape over your shoulder. Note the measurement. Add to this number.