1. Take a picture. Whole body shot is better.
2. Use Adobe Photoshop or GIMP. But I preferred GIMP.
File > Open As Layers...
Choose the deserved first layer.
4. Erase the sides of your model's body. You can use mask or select tool.
5. Add more pictures as many as you can. Do the Forth process to each picture you add.
File > Save As...
name your picture