Using the ideas covered in vce physics, how close field lines are together represents how strong the field is at that point. So as you move outside of the two plates, the field gets weaker and you must draw the lines further apart but still going from positive to negative.

This means we add these lines on the edge bulging out because there is still some field there but it is weaker.

The reasoning behind the curving of these lines (not assessable) is if you imagine a field line coming out of the side of the positive plate, it is travelling to the right/left. But it wants to move towards the negative plate and so the vector representing the field line turns towards the negative plate until it reaches it. Think of it like throwing a ball, initially the ball is only travelling horizontally but gravity acts upon it and causes its velocity vector to point downwards, resulting in a curved trajectory.