Drawing a "X" inside a box without lifting your pen or crossing any previously drawn lines is an impossibility. However, if you add a triangle at the top, it is fully possible.
To make this shape, begin by drawing a line vertically towards the top of the paper. Next, draw a line diagonally to the right and down. From there, continue the line in a vertical movement toward the top of the page. You should have a "N" shape drawn now.
Continuing your motion, draw the line horizontally to the left. This closes the top of the box. From the upper left corner of the box, which is where the pen should be, draw a diagonal line towards the upper right until you reach the midsection of the box. Then, continue your motion diagonally down and to the right, creating a roof on the box.
Next, draw a diagonal line to the left and down. This completes the "X" shape in the middle of the box. Finally, draw the bottom of the "house" you have created, as you have created a shape that allows you to place a "X" inside a box without lifting the pen from the paper or using the lines previously drawn.