Open your chat with the key on your keyboard. Write /fill and then get your coordinates by pressing the TAB key or press F3 and look at your X Y Z. Press ENTER (Don't worry on your chat about this "Usage" stuff). Then go diagonally to where you wnat to fill. Open chat again, then press the UP ARROW key on your keyboard.
That opens your previous command.

Get the coordinates for that block. Last, put in the block ID for what block you want to fill with. Press ENTER, and your done!