At least for me it is not quite clear what you mean by "Approve/Reject functionality".
Have you check packaged applications.
I think you can find something useful some of those apps.
i want to implement leave approve/reject functionality.
What you have already done for this?
I think nobody can explain all details to create complete application in one forum post.
Please start e.g. looking packaged applications.
Then if you have problem, ask specific questions and it is always good create example about problem to apex.oracle.com so we can see it.
In new questions always mention e.g. APEX version you are using.