Visit the Modal Verb Tutorial at EnglishPage.com. This site has seven different lessons on modals and online exercises to help you see how well you understand a particular modal. The end of the tutorial has a final test to measure your overall understanding of how to use modals.
Practice correcting mistakes in sentences that use modals. A list of sentences for which you have to choose the correct verb form is at ECEnglish.com.
Work through some of the dozens of online modal exercises available from EnglishExercises.org. This site offers modal exercises created and uploaded by English teachers from around the world. They are short and offer feedback as soon as you are finished with the exercises. All of the exercises are free.
Write a short paragraph using modals to practice using modals that you studied in the online exercises. Having read the many sentences you completed in the online exercises, you will have a better detailed understanding of how and when to use modals. The topic of your paragraph could be "What I Could and Should Be Doing Right Now." A sentence in your paragraph that uses modals might be, "I should be doing my homework right now, but I might go to the party instead."
Ask an English-speaking friend to help you correct your paragraph. As you get more confident in writing using modals, write longer and longer passages and have someone correct them.