The first click into the dangerous region canīt harm you. But the following clicks should be chosen cautiously!
To choose a square use the button-lines that surround the region. The left and bottom line clicks determine the horizontal (bottom) and vertical (left) position of safety-marks. To remove a safety-mark repeat this procedure for the same coordinates.
If you dare to "open" a square, risking to be blown up, you should use the top (horizontalcoord.) and right (vertical coord.) button lines.