I think there is a lot of confusion between a test plan and test case.
Test Plan:Test Plan is a document consisting of scope,objective,approach,schedule of the testing activities for the specific project.It mainly consist of who to test?,when to test?,why to test?,where to test?
IEEE 829-1983 standard test plan consists of different fields like,Test plan ID,Introduction,Test Items,Features to be tested,Features not to be tested,Testing tasks,Schedules,Tranining and Environmental needs,Test Approach,Deliverables,Item pass/fail criteria,test suspension criteria,human resources.
For login screen,test plan can be created.But it is not important to create test plan for login screen alone.A test plan will be created for the whole project.For Login screen,test case's can be created and executed to find defects.
Test case is a document consists of Serial Number,Steps Description,Expected,Actual,Test Data,etc.A test case is derived from SRS,FDS,Use Case's etc.For login screen ,A user interface test case can be created to check the spelling and location of user name,password,text boxes and for buttons ok and cancel and any remaining items on that screen.
Input domain test cases can be created for User Name and Password.Techniques like equivalence partitioning and boundary value analysis is used to create test data.Test data is divided into valid boundaries and invalid boundaries.For all valid boundaries positive tests and for each invalid boundary negative test can be created