Yielding hindquarters, in its most basic state, is just your horse moving his hind end away from you with a physical cue on his side. Typically the fingertips are used to press his side just behind where a back cinch might sit on a western saddle.
Think of lunging like a game of Simon says and see if you can keep your horse’s mind occupied. As you get better, see if you can work towards a walk to canter transition or a canter to a walk. See how fast you can get a whoa from a trot or canter.
A lot of horses have opposition to backing up. Groundwork can help your horse learn to be more responsive and in tune to your requests.
We talked about yielding hindquarters above, yielding forequarters is another favorite. I typically start with asking just for one or two steps and then building that up to where my horse can do a full circle, very lightly, to either direction