What you are training
Panning is the cheapest way to make a mix wider and clearer, and the least practised. Two guitars fighting in the centre separate the moment one moves left; a keyboard that muddies the vocal clears up at forty percent right. The ear is remarkably precise about position when trained and vague when not, which is why so many home mixes are a pile of sounds in the middle. This drill plays a source at a hidden pan position and asks you to place it, in steps from hard left to hard right. Each round shows the true position next to your answer, so you learn what fifteen percent sounds like against thirty. Use headphones or a properly placed pair of speakers; the drill also exposes when your listening position is off-centre, which is a problem worth knowing about.
After this drill, the plan: The daily plan pairs panning with mono compatibility, so every placement you make is checked against how it collapses on a phone speaker.