1. Select the particular cell or range of cells that you wish to lock.
   
2. Select Data > Lock> Lock Cells. You can also right click on the selected range and click Lock Cells.
   
3. The Lock Cells dialog box appears.
   
4. Choose the collaborators for whom you want to lock the particular cell/range . Switch the Lock button from its default "Unlock" option. 
   
5. The Lock icon turns green near the specific collaborator once you lock those cells for him.
   
6. If you lock a specific range of cells, that particular range remains locked by default even for newly added collaborators too.
   

Note:

Locking cells and sheets is available only to owners and co-owners of a particular spreadsheet. If you have "Read Only, Read/ Write" or "Read/Comment" permissions, then you cannot use Lock Cells.

Lock a sheet

You can also lock a particular sheet to be made inaccessible to edit for other collaborators.

1. To lock a sheet
   
2. Click Data > Lock> Lock Sheet.
   
3. Lock the sheet to those collaborators for whom you wish to lock the cells. The sheet stays locked to newly shared collaborators as well.
   
4. You can also lock the sheet by right clicking the required sheet and using the context menu.
   
5. You can unlock both cells and sheets in the same way you had locked them. Select the range of cells that you wish to unlock and click the Lock icon near the collaborators to unlock the ranges or sheets.
   

1. Click Data > Lock > Manage Lock Settings.
   
2. The dialog box shows all the cell ranges and sheets that have been locked by the user for various collaborators.
   

A green lock icon  indicates that the particular cell range has been locked for that specific collaborator. Click on it to unlock that cell range for that specific collaborator.

View all the locked cells/range

1. Click Data > Lock > Highlight Locks or go to the View Settings in the side panel > Highlight Locks.
   
2. The cells that have been locked for all collaborators will be highlighted and visible now.
   
3. To remove the cell/range highlights, disable Highlight Locks. 
   
4. You can view the Manage Lock Settings dialog box by clicking the lock icon present in the range.