Choose the right books to get started. Make sure they consist of simple, small sentences, as well as pictures with a description underneath. If you are lucky enough, you may find stories that you have already read in English. This way you can use your knowledge of the plot to figure out the meaning of individual sentences from the context.
Don't turn to a dictionary immediately when you stumble upon a strange word. Try to understand what it might mean, judging by the previous and later sentences. Of course, in the end you must ensure its supposed meaning is correct, but this procedure will definitely "save" the word to your memory permanently.
Study the sentence's structure. Try to spot the words used to create a negative or interrogative sentence, as well as how the words' order in the sentence is altered when this happens. When you have decoded the procedure and see that it applies to other sentences as well, it will mean that you have learned how to create such sentences of your own.
Understanding a text is different from being able to use the language. Make sure you occasionally use the simple structure of the book's content to create sentences of your own. Instead of "The boy plays with the ball," try saying, "The girl plays with the doll," or even change the verb, saying, "The girl loves the doll." Creating simple sentences will not only help you expand your Spanish vocabulary, but also make the sentences' structure part of yourself.
Practice with your friends and family, ideally those who speak even a little Spanish. Since you don't have classmates to share your knowledge with, try to narrate parts of the book to a relative or friend in the third person, or even try to communicate with him, using phrases learned from your books. Your accent may be lousy, as Spanish pronunciation differs greatly from English, but books can't talk, so for the time being, focus on the content of your speech.
Be persistent. Even though results won't be immediate, learning how to speak through children's books is like imitating the language learning process of small kids. This way, whatever is acquired through personal observation definitely won't be forgotten, as long as you keep practicing.