If you know your grammar isn't good, do something about it.

Instead of warning someone that your spelling and grammar aren't good, get a Beta reader. I mean you know that you lack skill in these two subjects (And weren't paying enough attention in English class to know when to use Were and when to use Was) and you know enough to warn people about it. So why not do something about it?

Chibi Horsewoman
Advice!

Many authors will tell their readers that they may have poor English grammar skills and there is nothing wrong with having weak grammar, but complaining about having weak grammar does not improve one's grammar. If you know you have a problem, in this case, weak grammar, do something about it.

Don't be afraid to crack open an English grammar book, ask a betareader to proofread for you or look on the internet.

On the other hand, some of you may be thinking, "Why do I need to study grammar? English is my first language!" You may be surprised to find out that your grammar is not as good as you think it is. If you have ever had one serious reviewer tell you that you need to improve your grammar, it might be worth checking into.

The point is: your English may not be as good as you think it is.

A good reference book for English grammar is Grammatically CORRECT written by Anne Stilman. It is a concise grammar book divided into five sections: Spelling, Problem Words, Punctuation, Grammar and Style. It covers the basics of the English language.