Watch means to weight out or to ponder. The idea here is that someone is pondering evil and then choosing to avoid it. It means to seriously think about the path your feet are going to take - and choose to take them in a direction pleasing to the Lord. Sometimes we don't think enough about the path of our feet. We just act and go along with our lives. We don't take the time to think about where we're going - what we're doing - and where it is going to lead when things reach a conclusion.
We are told that if we will do this we will have all our ways established. Ways is "derek" which again is the Hebrew word that refers to our lifestyle - the way we're going in life. To have that way or that lifestyle established means to have it firm and fixed. It means our lifestyle is one that is upright and good. We will have a life that matters and just as important - one that will be steadfast in what pleases and honors the Lord. There is also a sense in which we are being told our lives will last - whereas the worldling and the wicked man will only be temporary.
When we look at the way of our feet, we need to know not to turn to the right or the left. This is assuming that our feet are on God's paths and ways and not our own. When we turn to the right or the left, we are choosing to depart from God's paths and walk in our own, the world's ways, or those of the devil. Which ever of these three are true, we will find ourselves in a way that is wrong - and ultimately a way that will hurt and cost us.
The only turn we should make in life is one away from evil. The word "turn" here is a Hebrew word that means to go away from something, to desert it, to quit, to keep far away. When we see evil we need to remove ourselves from it - depart - honesly - RUN! To hang around evil is to court its greater influence in our lives. When we see evil we should hide ourselves from it. The longer we remain in the presence of evil, the more likely we are to participate in it. Thus the wise man runs from evil.
Think about where you're going. It is important to do this - to consider our lives and examine them from time to time. It is too easy to begin to wander from the ways of God - too easy to take a path that leads us away from the Lord. That is why this Proverb helps us so much - it encourages us to think through where our journey reaches its destination. We think about the end of things. When we do this - we will see the wisdom of turning away from evil and keeping clear of the wicked one and his ways.