I'm not good in explaining stuff. In other words, I'm a lazy fuck.
What you need:
Adobe Photoshop, a mouse and a fully functional brain.
1. Open your image.
2. Select View > Fit On Screen
3. Select the pen tool.
4. Using the pen tool create a shape which looks more or less the same as the image below.
5. Duplicate the shape.
6. Select the duplicated shape then Edit > Transform Path > Rotate 180
7. :)
8. Using the move tool, drag the shape to the appropriate corner.
9. Repeat steps 5-8. Except this time, change the Rotate angle.
10. Tadaaa!
If you have problems with these simple steps, please consult a psychologist immediately.
Kidding,
I would be happy to help you.