Put yourself in the scene you’re writing about.
Let’s say your story includes a forest scene? This might be where your character comes to find clues to solve a mystery, to meet a stranger, to meditate on his problems, to find a lost child, or whatever your imagination has conjured up for your story.
Picture a clearing in the forest that you might paint (or photograph).
Think about the overall composition. Consider the sky, the trees, the forest floor. What’s in the background? What’s in the middle ground? And, what’s in the foreground? Look at the details. What kind of clouds are in the sky? What color are the clouds? Are they standing still or is a current sweeping them swiftly across the sky? Are storm clouds building on the horizon? Is an abandoned cabin nestled in the trees on the far side of the clearing? What colors are the trees? Are they evergreen or deciduous? Is it autumn and the leaves turning ochre, crimson, and copper? Are there flowers or mushrooms growing? Is there a dead log with a fox den in it?
What time of day is it? What are the light and shadows like? Is it early morning with pink reflecting off the leaves and birch bark? Is it evening with orange tones glistening off the grass and the granite rocks? What are the main colors, and what secondary or tertiary colors peek through the leaves or grass.
What shapes do you see in the composition’s elements? Now that you have the basic physical scene painted, what don’t you see? What’s hiding behind the giant oak tree? What’s lurking under the waterfalls? Who is in the cabin? Is there an animal skulking into the dead log? Are the mushrooms fresh or rotting? What happened yesterday? What might happen tomorrow?
Free-write everything that comes to mind until your mind’s river runs dry. Then rewrite using the parts that make your scene come alive. Of course, don’t go overboard. Use only the essential components that help describe or drive your plot and character.
Good luck and have fun! Oh, and if your visualization skills are lacking, scour the Internet for a suitable image and use it to help your descriptive writing.