Click the AutoSum button. Then, one at a time, click each of the cells you want to sum, typing a comma to separate each cell in the formula (e.g. =SUM(C8,D10,F8,H8,J8). Another option is to hold down the CTRL key (instead of typing the comma) as you are selecting the cells.
You can also drag to select if there are some contiguous cells in your formula (e.g. =SUM(C8,D10,F8:H8,J5)