How to add Save as Draft button functionality and users can modify their own items? 

1. Please follow this post for Save as Draft button functionality, thanks Dan Stoll for wonderful post.
2. Under Version settings, set "Content Approval" to Yes and set "Draft Item Security" to "Only users who can approve items (and the author of the item) ". This would help us to hide own draft item from others.  
3. Under Advanced Settings Set Item Level permissions to "Create items and edit items that were created by the user".
4. Now Build you Nintex Form by adding two Calculated fields and Save as Draft button.
5. Set 1st Calculated field Value 
6. Set 2nd Calculated field Value to  if IE 9]  endif] 

   userProfileLookup(Current User, "PreferredName")

   
7. You many hide both these fields under banner by using "bring to front" functionality.
8. Add Set as Draft button
9. Add rule and set on 'Save as Draft' and 'Save and Submit' buttons and set following condition - 
10. Save these changes and publish it. 
11. Create the workflow for the list.
12. Add 'Set a Condition' action and set condition 'If Draft field value is equal to Yes' 
13. If action value is NO add 'Set Approval Status' action to Approved.
14. Add End Workflow action.
15. Save these changes and publish it.