Open the list or library which contains the list item or library file for which you want to create unique permissions.
On the Library menu, click List Settings/Library Settings.
The Settings window opens, click Permissions on this List/Document Library.
It's important to note that lists and document libraries inherit permissions from the parent site by default.
To delete inherited permissions:
On the Permissions menu, click Delete unique permissions to set unique permissions.
Select active directory groups or users who should be removed, click Remove user permissions.
To grant permissions:
On the Permissions menu, click Grant Permissions.
A Share dialog opens, click in the Invite people.
Enter active directory group or individual's name. Matches will start displaying, click group or individual to select.
Click Show Options and uncheck Send an email invitation.
From the Select a permission level drop-down, select the appropriate security level.