In order to test FF, try to break and create FF is step by step way. Use logs wherever possible. In order to check the ESS_LOG_WRITE message, please go to scheduled process (Navigate->Tools->Scheduled processes) and try to search for the process. You can find the log file for that process.
Assign a FF to DFFs or KFFs : Are you asking, how to use DFF/KFF in FF. You can use the DBI for them.
If yes, please find a sample code
Formula
---------
DEFAULT FOR PER_ASG_MGR_MANAGER_ASG_ID IS 0
DEFAULT FOR PER_ASG_ATTRIBUTE2 IS 'DEFAULT'

CHANGE_CONTEXTS(HR_ASSIGNMENT_ID = PER_ASG_MGR_MANAGER_ASG_ID)
(

RULE_VALUE = PER_ASG_ATTRIBUTE2

)
RETURN RULE_VALUE

Which DBI to use: Please read the description of the DBI carefully. You can find the description whenever you are searching a DBI while creating a FF on right hand side of page.