Open up Google Sheets and open an existing spreadsheet or create a new one.
Find or add cells containing numbers and click and drag your cursor over those cells to highlight them
Find the Functions button in your toolbar, left-click for a drop-down and select "SUM"
Hit "Enter" or "Return" on your keyboard
Follow previous steps to open spreadsheet
Click an empty cell and type the = sign
This allows you to select cells you want to add to the formula
Click a cell containing a number, type the + sign, then click another cell containing a number you would like to add
Hit "Enter" or "Return" when finished=
Follow the same steps as "Addition", only instead of using a + sign between each cell, you type in the - sign
Follow the same steps as adding and subtracting, only this time using a * sign instead of a + or -