While description is good to help your readers imagine what is going on in your story, you can still be too descriptive.  for instance, if you are describing a new character, you can give their eye and hair colour, and some information about what they are wearing, but don't go overboard. Your readers don't really care what brand name the shoes your character is wearing are, or what they cost. also, you don't need to explain your character's personality, If you do it right that will be shown through your character's dialogue and actions. 

You should already know that when someone is reading they imagine what is going on in the story in their minds even giving the characters unique voices in their head. what the reader imagines probably won't be identical to what you were thinking of when you wrote your description, but why does that matter? sure you may have imagined your character wearing a black leather jacket while someone else might imagine it as gray or brown but really what's the big deal? The story isn't about your character's clothes, and long descriptions get annoying and kind of boring since your readers are more interested in your story's plot.

Basically give enough description that your readers will be able to imagine what is going on, but not so much that they get long and boring. also, you don't have to give the description of a character or setting all at once, tell a little in one chapter, and if you think it's necessary, give a little more later on.  

here's an example.

 Jane had long blonde hair and bright blue eyes. Her dress was strapless and it went down to her knees. It was blue and was covered in sequins. There were so many of them that it sparkled like the stars in the night sky. She'd paid over three hundred dollars for it. She also wore black high heeled sandals.

I'm sure you figured out that description was longer than is necessary, for one like I already said we really don't care how much it cost. Here's a better one.

Jane had long blonde hair and bright blue eyes. She wore a blue strapless knee-length dress that had many sequins on it and her shoes were black high heeled sandals.

This second description might prompt a slightly different mental image, but it doesn't really matter since descriptions aren't as important as moving your story forward. Shorter descriptions explain well enough and honestly look better rather than rambling on about someone's wardrobe or the scenery somewhere.

Now I'm going to talk about the best way to introduce backstory.

While backstory is interesting and often gives the reader more information about a character, or other past events that took place in your story's setting it's best not to reveal everything all at once, and there are more interesting ways of doing it than flashbacks or just including it along with character descriptions. for instance, if one of your characters has had something traumatic happen to them in the past you could reveal that through making them have nightmares about it. 

no matter how you choose to reveal backstory it's best not to reveal too much all at once. five paragraphs of backstory in a row is a bit much since although they most likely explain things that the reader was most likely wondering about, it also delays your story from progressing and sooner or later your reader will get bored of that and probably skip ahead to the end of the long section of backstory which might make them miss something important.  

I'm also going to contradict something I said in one of the previous chapters. It is perfectly okay if you start your story without writing down your plan for it before you get started. winging it, and writing down ideas when they come to you is okay. I know of at least one famous author who writes this way and that is Stephen King. I'm sure that you've probably heard of him even if you haven't actually read any of his books. 

I recently read his memoir, and at least some of what he said basically confirmed things I already suspected, including the two topics that I covered in this part.  I will mention some of his other advice in future parts but for now, I think I've said all I can for this one. Like I said before you don't have to listen to my advice, after all, I'm no writing expert, but I'm just trying to help and I hope you can find some success here on Wattpad and possibly off Wattpad as well if you ever decide to try and get a story published.