Select the cell where you want the total to appear
Type `=SUM(`
Select the range of cells you want to add
Type `)`
Press `Enter`
Use AutoSum by selecting the total cell and clicking `Σ AutoSum`
Drag the fill handle to copy the total formula to other cells
Use `=SUM(A1:A10)` to total a range directly
Use `=SUM(A1,A3,A5)` to total specific cells
Use `Alt` + `=` to insert AutoSum quickly
Check that the cells contain numbers, not text