He starts with testing where the patient rates their feelings such as sadness, anxiety, depression, etc. on a scale of 1-10 before every therapy session, then works through one of these events of bad feelings - what triggers it, why, trying different techniques to help get past it, and finishes a couple of hours later with the same test to see if the symptoms improved. Test-driven therapy, honing in on techniques which help people feel better quickly, and rejecting those which don't.
He works on distorted thinking patterns and practicing rebalancing them. e.g. when you feel bad in some way, some of these patterns might be happening: https://worksheets.ambrasta.com/wp-content/uploads/2019/12/p... can you notice them as they happen and use these approaches to change them when they happen: https://glassempty.files.wordpress.com/2014/07/10ways.png
His more recent work is on identifying and unhooking the thought patterns which resist change, which say "I deserve to be miserable" or "I should feel guilty it's my fault", or "my boss is superior so I have to feel inferior" and then developing techniques to change those.
It's a very practical kind of therapy - not ten years of talking, not digging through a lifetime of history, or making grand diagnoses, but what can help you feel better about a specific thing next week, this week, today?
Related, his Feeling Good podcast series - contains a lot of updated ideas and techniques since the first book was written - https://feelinggood.com/list-of-feeling-good-podcasts/