ok,generally if SRS document is clear,no need of UCD(use 
case document),otherwise we use srs,ucd and businessrules 
to write the testcases

Hi

first u study and understanding the fdds or srs
then u will collect the list of the use cases
then u study the normal flow n dependence n entry n exit 
criterias for thouse usecases
then
based on that information
u will starts the preparation of test cases

Test Cases are primarily derived from functional 
specification documents/business requirements 
documents.Sometimes test cases can also be considered as 
atomic units, where the same cannot be split further. 
Constructing effective test cases involves techniques such 
as equivalence partitioning and boundary value analysis.