Reddit reviews How to Improve Your Foreign Language Immediately

I have read over 350 books on language learning.

That having been said, this very short book is definitely the best (even after 40 years of reading and learning about languages, I still learned a lot from this short book):
How to Improve Your Foreign Language Immediately
https://www.amazon.com/Improve-Your-Foreign-Language-Immediately/dp/0989387003

The second "must-read" is The Third Ear: https://www.amazon.com/Third-Ear-Chris-Lonsdale/dp/988988870X

These books are also terrific:
Art and Science of Learning Languages
https://www.amazon.com/Science-Learning-Languages-Amorey-Gethin/dp/187151648X

Language Logic: Practical and Effective Techniques to Learn Any Foreign Language
https://www.amazon.com/gp/product/0978064100

How much study time do you have available per day? Have you ever learned a foreign language successfully before? Do you speak any other Romance languages fluently?

Assuming you can study at least two hours per day, I would recommend:

1. Get Assimil's New French with Ease with the CD, and do two lessons per day. Spend 30 minutes on each lesson, following whatever variation of the Assimil Dutch instructions pleases you. In 25 days, this will give you a good, basic intuition for how French works, and teach you some useful vocabulary. The nice thing about Assimil is that if you follow the instructions, it works well for almost everybody, and it produces solid results. If you want a grammar overview to go with Assimil, get Essential French Grammar, which is dirt cheap, focused only on the essentials, and an excellent complement to Assimil.
   
2. Since you need to speak very soon, get Benny Lewis's book, which has some good advice on efficiently mastering survival stuff and polite conversation starting very early on.
   
3. A week or two before you leave, skim How to Improve Your Foreign Language Immediately, which is the bible of dirty tricks for faking a better level than you have. Definitely do his "islands" exercise, and prepare 10 or so islands, getting them corrected on lang-8.
   
   If you think of yourself as a hardcore geek, and you're generally good with languages, there are also a couple of ways to boost your listening comprehension substantially in 30–100 hours.
   
   Total cost: Less than $100, plus some money for iTalki tutors if you follow Benny's advice. But expect to work really, really hard—faking intermediate French after 30 days is a bit like sprinting straight up a steep mountain with a heavy pack. You're trying to compress 350 classroom hours into a month, which means working very hard and efficiently.
   
   Anyway, if you can spend an hour a day on Assimil, and an hour a day on Benny's speaking advice, then you'll get some pretty useful survival French under your belt by the end of the month. Going further than that will probably require studying obsessively.

Note that trying to speak in a new language for the first time can feel like you are trying to juggle, ride a unicycle and sing an opera all at the same time. Very little of skills you need are yet automatic, so you have to consciously think about each of them.
As everyone has said directly or indirectly, to get good at speaking, you must practice speaking. But there are sub-skills to it that you can try to practice on their own:

1. Pronunciation
   
2. Grammar (e.g. conjugations in many languages)
   
3. Word choice / collocations / idioms / stock phrases
   
   Initially just the physical act of moving your mouth differently to pronounce the language takes concentration. Depending on the distance of your TL from your L1, this can be VERY significant on its own.
   
   When someone says they can't speak well, my response: how many full sentences have you spoken in your TL? Even just reading out loud?
   
   One way to easy your way into speaking:
   Find a list of non-trivial, but not too long sentences (say 4-7 words each) and say them out loud. Make sure a) you know what they mean and b) you know how to pronounce them... even better if you can get a native to help you early on with pronunciation.
   
   Say each one over and over until you are satisfied with your "fluency" with each sentence. Practice the flow of speaking the language for a bit without worrying about "remembering" the content. (Actually, just speaking canned sentences a bunch of times will likely cause you to remember words and phrases from them.)
   
   Once you have some comfort speaking material provided for you, you can work on altering it or just generating your own sentences. You will find a lot of sentences can be usefully reused with simple noun/adjective swaps, e.g. "Where is the ?", "Can I have more ?" This is the beginning of simple conversations.
   
   A very effective way to start this is to create your own language islands (with the help of a native or lang-8), see:
   https://www.amazon.com/Improve-Your-Foreign-Language-Immediately/dp/0989387003
   (No affiliation, I'm just a fan of this book.)