Left-click in the desired square and choose a number from the popup keypad
-or-
Point to the desired square and press a number key on the keyboard
Left-click in the desired square and choose 'x' from the popup keypad
-or-
Point to the desired square and press the space key on the keyboard
Right-click in the desired square and choose a number from the popup keypad
(For single-button mice, hold SHIFT or CTRL while clicking)
-or-
Point to the desired square and press a number key while holding SHIFT