Excel - 3D formulas

I cant understand the instructions in the intermediate manual

In Sheet one called IF Function I have a list of commission amounts (column G)

In Sheet two called Conditional, I have a list of totals (column F)

I want to take the commission amounts from sheet one and add them to the totals in sheet two creating new figures in column G - ie totals with commission added.

THe manual tells me to select the cell where the formula is to be entered: this is col G in sheet two. It then tells me to click on sheet tab of the first worksheet to be entered into the formula (sheet two)). I am then told to hold shift key and click on the sheet tab for the last worksheet to be entered into the formula - sheet one. At this point, the new formula in sheet two, col g reads: =SUM('IF function:Conditional'!

I am then told to highlight the cell on the sheet I have just selected - sheet one. I then go back to sheet two and the formula just says =SUM(Conditional!G6. The IF function name has disappeared. G6 is the cell ref in sheet one but G6 in sheet two is where the formula is. I then completed the formula to read =SUM(Conditional!G6+F6) but it didnt work as the IF function (sheet one) no longer appeared in the formula.

I assume the formular should read: =Sum('IF Function:Conditional'!G6+F6). If I enter manually, an error message of REF comes up in the cell.