Recently, the amount of spelling and grammatical errors in instances has become increasingly common. We now require instances to be spell-checked before they are sent to the game planners.
It would also be incredibly helpful if instances are fully edited to the best of players' abilities. Knowing some have a language barrier, as well as different levels of education, we're not expecting perfection -- simply an effort. It's not a big deal if things are missed, as we'll also proof read before posting to fix any errors.
Generally, it's best to be short and concise to get your point across as well as spare others confusion. Editing helps in this because it's a way to clear up anything that could be baffling.
If you're unsure about something, The Elements of Style contains basic grammar rules with examples of proper usage. It also has a list of commonly misused and misspelled words. Unlike The Elements of Style, this list contains some tricks to remember how to spell some of those commonly misspelled words.
For those interested in accents, these articles contain some tips on how to make sure they don't become too difficult to read/understand.
Anything that can be done is greatly appreciated, but it's not something to stress out about. At the very least, simply remember to spell-check.