Custom workflow Creation

I need to create custom workflow for lead entity . In lead form field est.close date has been change frequently i will trigger my workflow for remainder and escalation mail in between the est.close date. Every time the est.close date field change new trigger happen and it will wait for execution till time reach . so If multiple changes occurs with in a day means i am receiving N number of mail . So my requirement is i need to delete all the existing process for the record when the est.close date field change and create a new process. Can you please  send me the entire code for this process because never been worked with CRM coding please some help me.