when should I add 兒 after words?

儿 it means samll an cute.ex: 女孩儿
it can change verb,adj to noun ex: 枕儿、团儿、尖儿

As I remember, once a Chinese teacher told us that sometines it can change the meaning of a word, and sometimes doesn't.
You can also search about that more.

Example:

儿 changing the meaning:
头(脑袋)——头儿(领头的；首领)

儿 nearly doesn't change the meaning:
事＿ ＿ 事儿

You can also look at here fore more explanation in Chinese:
https://baike.baidu.com/item/儿化/5667298

Hope this can help you!

You don't have to use it. It is not necessarily needed

Kind of hard to list all the words that need to add 兒. In fact, 兒化音 is being used less and less in mordern standard Chinese over that past decades. Most commonly seen in Nouns. Whereas there are some certain words that definately need to add 兒.
1. Where it would differenciate the meaning of the words. Eg. 頭 (head), 頭兒 (leader, more commonly seen in the oral and Northern China dialects)
2. Where it would differenciate the part of speech of the words. Eg. 畫(v. draw) 畫兒（n. drawing); 堆（v. heap）, 堆兒 （n. a pile of.../a stack of... ) Therefore, definately use "畫畫兒“ when they get toghether.
3. Where it may indicate intimacy Eg. calling the name of one's own puppy or children; 小河兒 （little stream)
Other than the above, most of the time, it won't affect the daily communication whether to add 兒 or not.

it is 1. a use of dialect in BJ Area and northern china
2. after noun to indicate intimacy or form a new word; after a verb, etc. to form a noun; etc.

It is kind of a dialect in northern parts in china.
They say 什么事shen me shi?-->shen me sher?