We tend to underestimate what we can accomplish when we add a little of something every day. What happens when you add one Spanish word each and every day? At the end of a month, you’ll have thirty new words. Maybe that doesn’t sound like much, but it is. One new word a day is more —much more— than zero words a day.
Adding just one word works because it’s easy. It doesn’t take a lot of time. It’s not something hard to commit to, such as adding fifty new words a day. The bottom line will be positive, in any case.
Let’s say you decide to add one new Spanish word a day. The question is: ‘Where do you add it to?’. I’m a fan of Anki, the spaced repetition software, so that’s what I recommend. But you can use paper flashcards, a notebook, or whatever works for you. As long as you regularly review your new vocab, you’ll be good.
Usually, it’s best to learn and memorize new words in context. That means you will add a whole sentence (it could be more than one, for extra context), including your new word. A good idea is to pick at least one sentence from the books you are reading in Spanish (I do it with fiction books to keep it fun!). If you read on your Kindle, you can highlight the phrase and —at the end of the day— go to the Notes section and copy the sentence into your flashcard system (digital or analogical).
If you feel like it, highlight and add more than one sentence a day (for example, three or five). Just remember that the minimum you’re aiming for is one word/sentence a day. That keeps it easy and doable. Anything else is a bonus.