The first thing to accomplish, is proper weight in your heels. Without your heels doing their job, you will feel like you are just "perched" ontop of your horse.
You have to rethink your approach on this though, instead of thinking "heels down" think, "weight in heels". That way, you wont end up forcing the situation, when instead, you will allow it to naturally occur.
In order for your weight to disperse into your heels naturally, you have to allow your bodies weight to flow from your head, into your seat, into your lower leg and finally into your heels. For this to occur, you cannot grip, pinch or tense up in your body, especially your knees. The moment you block that weight flow from occurring, your heels cannot do their job, which is anchor you into your tack. Your heels must be allowed to act as your bodies anchors.
You have to think of yourself as being wrapped around your horse, not just ontop of your horse. Open up your knees, and feel that weight flowing down. It doesn't matter if your heels are at a 90 degree angle, or a 40 degree angle - regardless, just so long as they are acting as your bodies anchors.
Another important factor, is to ensure that your feet are in your irons correctly, to allow your lower leg to be functional. The base of the iron should be at the ball of your toes, where the outer bar is at the tip of your pinky toe, and the inner bar is placed at the ball of your big toe.
That way, your ankles can relax and flex and move like hinges.
Once you have obtained a functional ankle and heels, you can then focus on where your calf is placed on your horses side. Again, you are wrapped around your horse - opening up your knees, sinking into your heels and ensure that your calf placement is correct - you don't want to have contact on your horses side with the back of your calf, nor do you want to use the direct inside of your calf....find your "Sweet Spot" - where your toe angle isn't out like "Charlie Chaplin" and you don't want your toes pointing directly forward.
THEN...lol, you want your lower leg under you. You want to be balanced over your feet - then, and only then, can you achieve a solid, functional two point position :)
Every upstride your horse makes at the canter, your lower leg is asking your horses back to come up into your seat. "Come up to me".
When moving into your two point, for cross country, firstly and most important, is being balanced over your feet. Of course, lower leg doing its job as explained already above - then getting your seat slightly out of your saddle. Enough where you can get a hand underneath. Tall Upper Body, acting as though you are lifting your heart up. Tuck your seat under you slightly, enough to straiten your lower back, and activate that core. Without your core working, you will not have that solid foundation to remain structurally solid.
When you achieve a solid and functional two point position, where all your body parts are put together properly like a puzzle - you can then achieve a balanced canter, forward canter, coffin canter, hand gallop. You will beable to chase your horse out infront of you and instantly bring them back under you - all using your core and body.
I hope that helps some :)