A couple of tips:
Set up your first sheet with all your formulas and formatting. When you`re happy with that one simply copy it. (Right click the shhet tab and select MOVE OR COPY.) A duplicate sheet will appear - simply double click it to edit the name to MASTER & press ENTER KEY. Now you should have a blank sheet that you can copy again and rename to the months you want.
Carrying the balances is even easier. In the cell you are bringing forward your figures to type
= then using your mouse click the previous sheet tab, select the appropriate cell and press the ENTER KEY again. The formula will write itself and the figure from the previous sheet will appear.