Informal mindfulness is when we, at any point in our day, become truly present and allow ourselves to be a witness to our thoughts and emotions.
You can practice informal mindfulness by "catching" yourself thinking.
Whenever you can remember to do so, pull yourself out of the "movie of the mind" (your thoughts), and ground yourself in the present moment.
You can do this by noticing present-moment physical sensations (internal sensations like your breathing or external sensations like feeling the support of the floor or or chair).
Just notice your thoughts as a third-person observer, realizing they are just thoughts and not what's happening in reality right now.