for ex 500 test cases are there.
we want to conduct retesting only 100 cases.
on the time we can introduce QTP.

1). After that each and every test cases created seperate 
actions. 
2). each and every action make re-usable action.
3). when ever u need the action on the time call the action.

Its upto the QA tester to analyse the conditions and decide 
on which tests to automate. Normally the automation comes 
handy when we perform regression testing (ie . once system 
is stabilised and new features are added then we perform 
regression testing to make sure the existing features work 
properly. In that case its easy to automate the testing. 
Also to check some calculations adn user interface we can 
automate. The order of execution is totally based on the 
application and its difficult to point out exactly.