The front of a pack is advertising. The back is closer to the truth. You do not need to memorise anything; you just need to know which few numbers actually matter.
Sugar and fibre, together
Look at the carbohydrate, the “of which sugars”, and the fibre. A food with a lot of carbohydrate and almost no fibre has usually had its original structure stripped out, which is part of what makes it easy to overeat. Fibre is one of the things that tells your body it has eaten.
The ingredient list order
Ingredients are listed by weight, heaviest first. If sugar (under any of its many names) appears near the top, or several different sugars are scattered through the list so none of them looks dominant, that is worth noticing.
Per 100 g, not per serving
Compare foods using the per-100 g column, not the per-serving one. Serving sizes are chosen by the manufacturer and are often smaller than what people really eat, which makes the numbers look better than they are.
Educational only. Not medical or dietary advice. If eating feels out of your control, speak to a qualified clinician.