Giving Document level permission in a library

If you want to give separate permission to a folder inside a document library then do the following:
Go to the site settings --> advance settings.
Create a new Permission level and choose permissions which are not available previously with other permission levels.
After doing it go to the folder and choose 'Manage Permissions'.
Add the users you want to give access to . Edit Permissions of the selected users. Select the permission level just created.
Remove all the other users whom you do not want to give access to the folder.