Nice question
To know when you start entering sell/buy orders you have to know first the trend and the support and resistence levels for each currency
try to buy around the support level if you think the trend is neutral and no upcoming events that would send the trend significantly lower
The stop loss usually should be placed around 20-30 pips below that, but don't worry if once you start the order the level goes 5-10 pip lower as the market fluctuating but if it market get lower quickly to -30 pips then consider using the stop loss especially if the fluctuation hit the lower level several times and unable to go back to the level you bought it with
The limit order is usually just below the resistence level or if you see the market clearly passed that resistence level, the best thing is to wait and watch any news carefully which would take the market hight
The worse thing is to be not patient and not able to get the profit
Good luck
