1. Select the cells that users will modify during data entry.
2. Go to Format > Cells, deselect the Locked checkbox on the Protection tab, and click OK.
3. Go to Tools > Protection > Protect Sheet, and click OK.

Now users will be able to alter only the cells that are unlocked. If users attempt to alter any other cells, they will receive a message saying they must unprotect the cells to modify them.

In Step 3, you may wish to add a password to prevent unauthorized users from changing a cell's protection level, rather than accepting the default settings.