Hello, Olivia! Here are your corrections for today...

    ***********CORRECTED DIARY OF OLIVIA PARK**************

I went to Morning Glory, which is a toy shop in my town. I bought stickers with 3 characters. Also, there there was a lot of colorful stickers used to decorate. My favorite character has pink hair and she has beautiful clothes. I put dandelion stickers on my favorite character's hair. It was so fun.

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OLIVIA: I went to morning glory,which is a toy shop in my town.
CORRECT: I went to Morning Glory, a toy shop in my town.

EXPLANATION:

a. Please capitalize the proper noun.

b. Let's try our best to minimize unnecessary words. In your sentence, there's no need to put "which is" because it is already understood.
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OLIVIA: I bought a sticker.The stickers had 3 characters.
CORRECT: I bought stickers with 3 characters.

EXPLANATION:

a. Because you told me that there are 3 of them, let's use the plural form of sticker which is STICKERS.

b. Instead of making 2 sentences, let's simplify it as one using the preposition "with."
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OLIVIA: Also,there there was a lot of colorful stickers to decorate.
CORRECT: Also, there  was a lot of colorful stickers used to decorate.

EXPLANATION:

a. Oh, be careful, there's a repeating word; a typographical error.

b. Put a space after a comma <,> or a period <.>

c. The sentence would be clearer when we add the word USED. We can also say: There  was also a lot of colorful stickers for decorating.
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OLIVIA: My favorite character had pink hair and she had beautiful clothes.
CORRECT: My favorite character has pink hair and she has beautiful clothes.

EXPLANATION:

Use the present tense "HAS" and not the past tense "had" because until now your favorite character has pink hair and beautiful clothes, it is not just before.
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OLIVIA: I put dandellion stickers in my favorite character's hair.
CORRECT: I put dandelion stickers on my favorite character's hair.

EXPLANATION:

a. Be careful with the spelling. 

b. Please use the preposition on because you attached the stickers on the surface of your favorite character's hair --- not INSIDE. ^^

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Good job, Olivia! Oh, I'm wondering whose your favorite character. ^^

Keep up the good work and don't forget to study about the explanations. Thank you. See you tomorrow!!

Lots of love,

Teacher Ace^^