I keep losing the far-post winger when crosses come from the other side

Your first job is to arrive at the back-post area early enough that you can see both the ball and the player. If you stand square to the cross, you will usually lose one of them. Try to recover with your body slightly open so you can keep the winger in your peripheral vision while still reading the delivery. As the ball moves to the opposite side, narrow with your centre-backs rather than staying attached to the touchline. Then adjust your distance from the winger. Too tight and they can spin behind you; too far and they can attack the ball first. You want to be goal-side and close enough to affect the run once the cross is actually coming. The key habit is checking the winger before the cross is delivered, not waiting until the ball is already in the air to find them.