Select the cell where you want the total
Type `=SUM(`
Select the range of cells you want to add
Type `)`
Press Enter
Example: `=SUM(A1:A10)`
To add separate cells, type `=SUM(A1,A3,A5)`
To sum an entire column, type `=SUM(A:A)`
To sum an entire row, type `=SUM(1:1)`
Use AutoSum by selecting the total cell and clicking `AutoSum` or pressing `Alt` + `=`