Fnctional Testing starts with the through reading of 
Functional Docs, and getting to know how of Use Cases. This 
then comes up with Development of Test Cases, and Executing 
them. In case of Time Constraints, One can go for Monkey 
Testing too.

Functional Testing begins from Functional specification 
requirments(FRS)Docs,understanding the functionality of the 
application with the help of Use Cases(If application is 
not yet developed),Start writting test cases and exceute 
the test cases.
Actual functional testing means verifying the dependent 
field on depedency field.
Example:once the user information is filled with all the 
mandetory field then only SUBMIT button should be enabled
(i,e Here SUBMIT buton depends on dependency fileld)

Generally Functional Testing starts, once we are prepared 
the testcases for all the features as per the requirement 
specs.While preparing testcases,we need to cover all the 
+ve testcases,-ve testcases,adhoc testcases and GUI 
Testcases and also we need to assign certain priority to 
the testcases.As per the testcases we will perform diffrent
levels in Functional Testing like FULL and Medium.We can 
also perform Sanity Testing as per the Testcases priority.

Full Functional Test Pass:
...........................
Should be executed all the testcases from P1 TO P4.

Medium Functional TestPass:
.............................
Should be covered only P1,P2 and P3(Optional)

Sanity Testing:
----------------
Should be covered all the P1 Testcases.

Note:All the above types of testing techniques as per the 
deadlines of our project.