This is a simple Expert Advisor with an implemented trailing stop.

It has only two parameters:

TrailingStop — the distance between the price and the Stop Loss level.

TrailingStep — protection against to frequent modification of position. 

How TrailingStop works — the example of the Buy position: 

Originally the StopLoss of a position is equal to zero, we wait for the following condition to be met

— i.e. when the Bid minus TrailingStop is greater than the position open price PriceOpen:

StopLoss is moved to the PriceOpen level (position open price):

now, when the position has StopLoss (i.e. when StopLoss is not equal to zero), wait for the following condition to be met:

— i.e. when the Bid minus TrailingStop minus TrailingStep is greater than the StopLoss price: So here we need to additionally check TrailingStep — this prevent position modification on every tick:

StopLoss is then moved to Bid minus TrailingStop:

Here is why we need a protection in the form of TrailingStep: