There is a popular misconception that creative writing, as a creative process, is above criticism and academic rigour. This is utterly false. Criticism is absolutely necessary to improve any work, and the imposition of academic standards can greatly improve the quality of a work. This being said, as the author you have complete control over your work. If you feel that the violation of grammatical conventions improves the work, do it. Creative writing also requires dedication. You must be willing to put time into a work, to edit it, and allow it to be edited. Beyond these, few absolutes can be drawn. Different people write in different ways. You need to find a method that works for you. Finally, the broader your knowledge of the language and the more that you have read, the better your writing will be.
Some people can simply sit down to write and have something to write about. For others, finding something to write about can be the hardest part of creative writing. Assuming that you are not in the first group, there are several things you can do to create ideas. Not all of these will work for all people, but most are at least useful tools in the process. Also, you never know when you might have an idea. Write down any ideas you have at any time and expand on them later.
For stories and poetry, the simplest method is to immerse yourself in the subject matter. If you want to write a short story, read a lot of short stories. If you want to write a poem, read poems. If you want to write something about love, read a lot of things about love, no matter the genre or medium. This method can take a long time for people. While it often produces 'inspired' works, it can take a long time to do it.
You can take a different work that you like (preferably a long one) and start to copy it by hand until you reach a point at which you feel you would say something different from the actual author. At this point, start writing that and discard the other work. You can go back and change the beginning later. This can be excellent practice, but it obviously produces somewhat derivative work.
You can do a simple procedure in which you pick a word, phrase, object, name or something at random and start writing about it. This is also a good exercise, and it can produce excellent poetry. However, some people have just as much trouble picking a word or writing once a word is picked as they have coming up with an idea.
Once you have started a work and have begun to write in earnest, you should remember certain universal writing tips, such as making sure that you write in a non-disruptive environment. Some people find that mapping out what they are writing ahead of time, or preparing rubrics is helpful. While this can help focus your writing, other writers find it restrictive. Experiment to see what is right for you. Most importantly, revise your work and keep copies of the various revisions. If you start to feel that the work is complete, let it sit for a few days and read it again. You will be amazed at how many changes you will want to make. Keep in mind the importance of language and expression, as how you say it can be as important as what you say.
Once you have finished a draft of the work and feel it is ready, you should have other people criticize it. Make sure they tell what they think and why. Decide if you agree with them, and then act on it. Do this several times, with many people. There are websites which can help you to do this. Remember, you have no reason to make a change unless you agree with it.