My favourite way of teaching them correctly and to get the right feel is riding a 15m x 15m square always in shoulder fore with a 90 turn on the haunches/pirouette in each corner. Keeping them in shoulder fore keeps you in control of their shoulders and keeps prompting the idea of keeping them bent in the direction you going. So when approaching the corner, you keep their head and shoulders slightly to the inside and then push their body a 90 degree turn while keeping the slight banana shape. Practice doing them very slowly so you can get them to think about each step they're taking and for you to have the correct feel of the shoulders moving around the hind end. Keep up the forward thinking so that your horse is less likely to cross over his hind legs (big no no), doesn't get stuck and has the mentality to move forwards, that his 'escape' is going forwards.
This can then later down the track be transferred to canter pirouettes where you need all the go and all the sit at the same time. If they don't think forward you will never get the lift and neither the sit.
For your seat, keep your hips and lower back really elastic so you don't block any of the movement. If anything; slightly swing your hips in the direction you're going with each step which will help keep their back soft and not lock on the contact. Also turn with your hands and shoulders too. Not so much that you or someone on the ground can see, you want your hands to look like they haven't moved an inch, but more of a feel to keep guiding them around to your desired direction.
Hope this helps!!! If there's anything else in particular you need a hand with, don't hesitate to ask :D