MT4 Expert Advisor based on SMA crossover

I need an expert advisor programmed in MT4. It should check to see if an order is open. If no order is open it needs to buy when the 50 Simple Moving Average (SMA) Crosses above the 200 SMA with a trailing stop loss equal to 3 times the ATR value at time of order execution. It also needs to sell/short when the 50 SMA Crosses below the 200 SMA with a trailing stop loss equal to 3 times the Average True Range (ATR) value at time of order execution. The order should be for 20% of the account balance