Select the cell where you want the result
Type `=SUM(` followed by the cells or range you want to add
Example: `=SUM(A1:A10)`
Press Enter
To add individual cells, use `=A1+A2+A3`
To add across rows or columns, drag the formula if needed
To use AutoSum, click the cell below or beside the numbers and select the sum function
Make sure the cells contain numeric values
Use ranges with commas to add non-adjacent cells, such as `=SUM(A1:A5, C1:C5)`