Mastering it before moving on in grammar isn't a bad idea. Exposing yourself to (massive) amounts of native written text will help with the relation of words. I had the same sort of trouble when I started. The idea is that there are no floating words, that just have no specified job. Every word is exactly labeled for what it does in the sentence. I'm not even a native speaker, and I can attest that the concept of a word having a different ending depending on its job in the sentence (object, subject) fits very very well into the mind. It certainly becomes automatic, even obvious. Going over each case, and all of their reasons for showing up, should help. It's important to always know why a word is the case that it is in a sentence, because it's never an accident. Your brain notices and files away patterns until "Я вижу кошка" sounds wrong. Just like wrong grammar will "sound wrong" in your native language, this sense will come, and you probably won't notice it happening. Out of the parts of speech- nouns (pronouns), and adjectives decline. Ideally, any adjective will always have a noun to which it refers, and is tied to. An important concept is that genitive mainly requires another noun somewhere in the sentence that is affected (this helps with no word ever being left out). Exposure to fully written sentences is what will develop the sense of organization that declension gives. All the words in every sentence in any language are connected to all the others in some way, Russian just marks the connections. English sentences often just simply miss an object and we have no problem with that, but in Russian, every single word is specifically where and what it is for a logical reason.