You can start off by laying with your back flat on the floor with your legs straight, like with a regular sit-up. Bring up your knees to a comfortable height, keeping your feet together. Then proceed to drop your knees outward to the sides (they needn’t touch the ground, this is just to “switch off” the muscles connecting to your quadriceps and glutes), all the while keeping the soles of your feet together. This is the part that reminds me of a butterfly. This will take some getting used to, and once you start the movement you will quickly see what is most comfortable for you. The closer your feet are brought up towards your body, the more effective the movement, and you will see the stronger your core gets, and the more flexible you become, the closer you will be able to bring your feet.
You will then extend your arms backward touching the ground. Some people prefer to cross their arms across their chest touching their shoulders, which isn’t wrong, but extending backward increases the range of movement, helps with flexibility and gives your spine and back muscles a great stretch.
Then, with your legs all the while staying in the butterfly position, bring your arms up and use the momentum to do the sit-up all the way up and forward, and touch the ground in front of your feet. It is best practice also to control your breathing – exhaling on the “up” movement, and inhaling as you go backward.
You should aim for as many repetitions as you feel comfortable with, or you can use this technique in a circuit.