Don't bother with course materials I would say. Just speak Spanish with them as much as you can. Maybe watch a few Spanish-language tv shows, especially some intended for kids (Sesame Street?) since those usually have clear pronunciation.

Really the first important thing in a new language is to train your ear to break down the sounds of continuous speech, so it sounds like words instead of mush. That takes some tens of hours to get traction at all. So do lots of passive listening with your coworkers. Stuff like grammar and vocabulary come next and can be gradual. Course materials can help with grammatical subtleties but basic speaking and listening come first.

Also, try to quit speaking English quite early if your coworkers will put up with it. Speak Spanish and use a dictionary if you don't know a word. Don't bother with flash cards. You'll forget stuff and have to look it up again and that's fine. After a few repetitions it will stick.

Try to make your pronunciation very accurate. That will help in all areas. We think of phonetics, vocabulary (lexicon), and grammar as separate things, but they are all interrelated and mutually reinforcing.

You will find yourself thinking in Spanish instead of English pretty quickly. You are basically reformatting your brain so just roll with it.

Be aware that your English can actually get worse from this process. At least mine did. Decide if you are ok with that.