When you create a formula you can use the mouse to select the cells you want to use in that formula e.g if you want B3-F7 then (after selecting the '=' for the formula format) click on cell B3, type '-', then click on cell F7 and click on the tick.
The important thing for what you want to do is that those cells that you click on can be on another worksheet - the name of the worksheet is prefixed to the cell id.
I don't have Excel available to me currently to let you know the exact form of what gets put in the formula as a result - I think its 'sheet name':B3 etc.