Hope this helps!
Sit with your back straight, and let your legs hand down slightly behind the girth. Tip your toes up and shorten your stirrup leathers until the stirrup is just above your toe. Flex your knee slightly so that your toe slides into the stirrup, but your heel is still below your toe, keeping an easy comfortable contact with the stirrup. Your leg should still be relaxed and you should be sitting on the horse, not half-standing in a crouched position, waiting to have to post. In case you haven't already, now is the time to relax. Sit quietly on your horse, until you feel all the tension ease out of your body, and you are comfortable.
Balance is maintained more with the upper thigh, than with the knee and the calf. Apply no pressure with your lower leg unless you are cueing for a gait change. It is not necessary to hold onto the horse with your legs, as these gaits are very smooth, and do not dislodge the rider.
Your hands should be held so that there is a fairly straight line from your elbow to the corner of the horse's mouth. Keep your hands soft enough so that when the horse nods his head as he is doing his gait, the natural action of the neck shakes your hand up and down like a handshake. Do NOT attempt to set your horse's head by cresting it at the poll, as you will shorten its stride and reduce the comfort to both horse and rider. Take only enough contact on the reins to feel the bit, and be ready to give more rein to accommodate the depth of your horse's nod. Do NOT attempt to pump or pull its head into a more exaggerated nod that it is producing naturally. Most Tennessee Walking Horses are ridden on a direct rein, although they can be taught to neck-rein just like any other horse, if you prefer.
Sit back and enjoy the ride