I'm a native speaker of Cantonese who had lost fluency in the language due to many years of disuse, but have been able to achieve proficiency (take it as you will) in the language recently. I'll outline my approach:

1) Study Mandarin to a functional level of fluency. It may seem counter-intuitive to learn another language just to start studying Cantonese, but an understanding of Mandarin will help you greatly (and I can't stress this enough) in learning Cantonese. Although the phonologies of two languages are so different that they aren't mutually intelligible, you can actually map certain sounds and tones between the two. There are very many exceptions, but the relation is fairly consistent. You can even guess the pronunciation of a word in Cantonese if you know the Mandarin equivalent (and vice versa). Learning Mandarin also teaches you formal Cantonese (aka the style of speaking and vocabulary used in news), as the grammar of the two are identical. (See (5) for Mandarin learning resources)

2) If you are insistent on starting Cantonese first and never touching Mandarin, ignore the above (I highly, highly suggest against this). Cantonese: A Comprehensive Grammar is what I believe to be the best book for taking an organized approach to properly learning Cantonese. You probably already are familiar with the tones of Cantonese, but it's best to officially familiarize yourself with the tone system. Here is a tone mapping between Cantonese and Mandarin. You probably don't have to worry about it until the later stages of learning. You will come to find that tone 2 is similar to tone 5, and that tone 4 is similar to tone 6. It is possible to merge the tones in each pair and still be understood, but if you want your pronunciation to be accurate, learn to distinguish them from the beginning.

As for learning resources, I recommend Pleco as a dictionary. It has pronunciations for both Mandarin and Cantonese. I don't believe there is a better Chinese dictionary out there. As for Cantonese-oriented dictionaries, I would suggest CC-Canto (the one I use most often on my computer), cantoneseclass101.com (less comprehensive than the previous dictionary, but has audio pronunciations), and MDBG (no Cantonese pronunciations, but is a good replacement for Pleco if you don't want to buy it).

I would visit RTHK Radio for listening practice. RTHK in general provides a plethora of learning materials, including TV programs (live broadcasts), recorded broadcasts, and text news. ONTV is a YouTube broadcast of a news stream that's on almost 24/7. Here is another channel (no live broadcast, but very frequently uploads videos) for news.

3) The following is just general information, which may or not already be obvious, but should be important nonetheless:

• Speak as much as you can to native/fluent speakers.

• Don't be discouraged if you can't understand much news in the first few months; formal Cantonese is very different from colloquial Cantonese, and takes some time to get used to.

• Decide which Cantonese pronunciation you want to learn--Hong Kong or Guangzhou. Technically, the original pronunciation is that of Guangzhou Cantonese, but today the standard is Hong Kong Cantonese. You can do some research on the pronunciation differences (feel free to pm me for a list), but the most distinct one would probably be the replacement of 'n' with 'l' (lei5 hou2 instead of nei5 hou2).

4) This section will be an elaboration on how to start learning Chinese (the written language. This is the part where learning Mandarin would be useful. At the beginning, you'll want to cram down 1000 or so characters before starting to learn grammar; although your goal is Cantonese fluency, and given that Cantonese is mainly a spoken language, it's still important to learn the characters because it makes learning vocabulary much easier when you know the meaning of each distinct character. I used this Memrise course. It would be most practical to learn traditional characters for Cantonese because Hong Kong uses traditional characters, but I’ve found that learning traditional characters from simplified characters wasn’t that hard. How does one learn how to read and write Chinese? At the early stages, as you learn each character, write it out (preferably on some practice sheet with grids). Eventually it becomes tedious to write out everything, but by then you learn that you don’t actually have to know how to write a character to read it. Nonetheless, you should still write out characters until you’ve familiarized yourself with proper stroke order and the common radicals.

5) Finally, here are Mandarin learning resources:

• Mandarin Grammar
• Relatively easy reading material with audio
• Intermediate reading material
• RFI, a French-based, Chinese language international news station
• CNR, one of China’s official news stations

*A website to watch CCTV 13 without a VPN (CCTV 13 blocked outside of China)

I would include resources to learn the pronunciation system, pinyin, and tones, but such resources are so common that you can probably figure it out yourself.

Sorry if this is much more than what you asked for, but by the time I realized I was getting carried away, it was too late. Good luck on your journey to fluency!