There has been a lot of research over the past few decades on the use of extensive reading for language learning, with Paul Nation being a prominent name in the research community.  Out of all this research has come some general guidelines on how to use extensive reading to improve your language learning skills, but also how to write or adapt stories to suit language learners.  Here’s my version of the basic requirements.

1. Decide what your core vocabulary will be, for example 1,000 word families.  You may also want to decide what grammatical repertoire you are going to include – at least for lower levels of language skill.
2. Decide whether you want to teach a particular set of vocabulary in the story (eg. colours).
3. Ensure that at least 95% of the text consists of words from your core vocabulary or proper nouns.
4. For words in your original draft that are outside of the core vocabulary, consider changing them to ones that are within the core vocabulary.
5. For the remaining out of vocabulary words that occur less than 5 times (say) in your story, provide a gloss.  (Also for any idiomatic expressions.)
6. For words that you want to teach, ensure they occur at least 5 times in the story, but in a way that doesn’t ruin the story.  It would be better to have fewer occurrences than to make the story less entertaining.
7. Use illustrations, as they help the learner retain meaning.

Here’s a vocabulary checker for stories in English.