convert custom indicator to EA (expert advisor)

Hi .. I have a custom indicator that gives sell and buy alerts. I want to convert it to EA that convert these alerts into orders as follows " @@ when a buy signals comes, it closes the sell order and opens a buy order. @@ when a sell signal comes , it closes the buy order and open a sell order. depending on these two rules above : @@ the the maximum opened orders at the same time is always only one trade. @@ there is always a trade opened. @@ I have to be able to set a default lot size, SL , TP to the orders. note : when the ea is launched it must check if there is order opened or not, to make sure not to open more the one order at the same direction. ## what I need is to convert the code itself from an indicator code to EA code ( the use of icustom function is not allowed ) ## you can contact me if any further information is needed .