At this point you should be able to yield both the hindquarters and the front end in both directions. Now we are going to put both exercises together in what I call Hind/Front.
Lets start with the horse tracking left on a circle around you (at an active walk, of course). As you yield the hindquarters, the left hind leg will cross over the right hind. After a couple steps of hindquarters pause and allow the horse to pause and rock his weight back onto the hindquarters then yield the front end to the left (left front is the leading foot, right front crosses over) until the horse is positioned to track right. Add some forward motion and the horse is now circling you to the right.
At the beginning it can be confusing for people organize in their mind which way they are supposed to turn. In the example above, as the horse is yielding its hindquarters it is looking (or being bent) left. It is going to continue looking (or being bent) to the left as it yields the front. If, after yielding the hindquarters you find yourself straightening the horse's neck and changing the direction it is looking, you are preparing to yield the front in the wrong direction! Remember that the horses stays bent in the same direction for both hind and front.
To start with let the horse complete one of two circles each time you change directions and focus on achieving Hind/Front. As this becomes more comfortable, change directions every 1/2 circle. Be precise about getting the changes of direction at exact points on the circle so you learn to control the timing of the maneuver. Lastly, practice moving your 1/2 circle towards a destination. You will be walking in a straight line with your horse moving on a 1/2 circle in front of you, achieving Hind/Front with every change of direction.