Ideally you want to tap while any emotion is surfacing. Ie, while you’re upset.
The closer the emotion is to the surface, the easier it is to tap away. This is why why we tap the collarbone point to get started – it helps bring everything up.
Since the primary requirements of tapping are just that have a couple of minutes of uninterrupted privacy, I’ve found it’s very convenient to escape out to a bathroom and tap, or when I’m having a shower in the morning.
It’s also possible to tap in public (eg when you’re walking around), by simply saying the phrases in your head, and imagining tapping each of the points in turn, rather than actually physically tapping. Alternatively, you can also tap on the sides of your fingers, as detailed here. These aren’t quite as powerful as regular tapping, but they’ll help in a pinch.
Because EFT is so fast, and so versatile, it’s recommended that you try it on any negative thought or emotion you experience. From getting angry in traffic, frustrated at work, or even a headache, a couple of minutes spent tapping will typically result in very fast relief. Of course, you can also use it in a more structured manner to tackle much harder problems (see “Tips if you’re stuck“)