Use your skills of observation. How do you read people in real life? How do you know if someone is angry, happy, sad, crazy? You interpret their words, their body language, their actions, their facial expressions. How do you know when it's cold outside? You see people with red noses dressed in warm clothing. They are clapping their hands together and bouncing in place to stay warm. They are shivering. Their breath steams the air when they talk. The wind is whipping the bare branches of the trees. Maybe there is snow on the ground. All of these clues paint a picture that show the reader "it is cold" rather than just telling them.
Let the reader experience the world through all their senses. How does something look, smell, feel, sound, taste? What does your intuition tell you? For instance, if someone eats something that tastes bad, how can you know that? They may grimace, they may spit the food back out, they may shudder, you may detect an off smell to the food, you may hear them gag. Describing these things shows the reader the food tastes bad rather than simply telling them so.
I once did an exercise for fun when I lived in a town where I had to go through toll booths on a daily basis. Each day I would study the cars ahead of me and try to guess what the person driving it was like based on the clues i had: the arm that extended out the window (feminine, masculine, adorned with jewelry, fat, thin, etc.) the kind of car they drove (old, new, minivan, sports car, pickup truck), the way they tossed or laid their money in the bin (did they throw it or toss it or lay it in, did they come to a full stop of coast through?), the way they took off after paying (did they slowly merge into the traffic, or gun it and try to take the lead?) Did their car rattle as it idled and expel nasty smelling fumes, or run clean and quiet? Using these clues I would determine the personality of the driver and develop a mental picture of what they looked like and the type of life they led. Describing what I saw in words would be a way of showing rather than telling.
Beth