A lot of Fanfiction writers just show a picture. Stop. Some readers may stop reading seeing this lazy annoyance that rips them out of the immersion. Not only is it just straight up lazy as fuck, it just stops the plot and rips the reader out. If you struggle to describe something, just practice. And again, read others writing, see and learn how they describe something.

Now, I would like to mention, that you can describe things wrong. Lets say you write something set in a fantasy world, unless the character is from earth, though dimensional travelling or something like this, don't describe via comparison to things in our world.

The next would be, don't reuse the same words constantly, it gets boring. Though, if you reuse a word, try to use it for the same thing. Don't describe everything with X. Try to describe only Y with X, and Z and W with G and V each. Especially when Y reappears later.

You describe monster Y having steps that sound like thundering, later they return and get reintroduced. "Kara hears thunderous steps." The reader realizes Y returned! Though, you could use this word exclusivity to throw the readers off a clue, if you write a mystery or something. Though, it doesn't hurt to describe one thing with multiple words, but don't describe it with every word you know. Good prose, and descriptions are often simple. Sometimes the beauty is hidden in simplicity and not overwhelming flowery writing. Sometimes less is more.

All in all, won't hurt to look up synonyms and to build a bigger vocabulary. Heck, though writing and reading fanfiction, I got better in school. From a 4 to a 1. 6 is worst, 1 is best. There were sometimes situations were I knew a word in English, but my teacher didn't!

Also, I noted that a lot of people focus on colour when describing. In most cases, colours aren't the main priority to describe.