Decide how many primary character people will be in your tale and by that I mean those who you will give a personality to. Who will be the one in charge - who will be the nice person or the mean person - the childish being - the one who will accept others view points.
A fan fic will always be improved if the reader can see who is the good guy and who is the bad guy ( this is a case of do as I say not do as I do ).
If you are writing about a known character ie Buffy, Willow, Giles adhere to that person in the series which can be difficult when attempting to get that character to do something other than was in the series.
Read through what you have written to correct mistakes in grammar and spelling. On a separate occasion read through what you've written to make sure it is easy to read and makes sense.
Give it some time before entering it as if you read it through straight away your brain may see what you've written as being correct without actually correcting any of your mistakes.
Make sure you use correct words, I.E., your, you're type words. Get to know what each of them means and use the right and wrong one in a sentence.
That is your pencil.
That is you're pencil.
Mistakes like them make the story hard to read as the reader will need to stop to try and understand what you are saying before continuing - affecting the flow of the tale you have worked so hard to be meaningful.
Sod the variation of English English or English American. Even if many folk who learn English as a second language will be taught English English. The difference is like a 500 page dictionary.