First, you have to define what you mean by "fluent". There are several proficiency scales that may provide some useful insight:
- Common European Framework of Reference for Languages (http://www.coe.int/t/dg4/linguistic/cadre1_en.asp)
- ACTFL proficiency guidelines (https://www.actfl.org/publications/guidelines-and-manuals/ac...)
- Interagency Language Roundtable scale (http://www.govtilr.org/)
Here is a quick-and-dirty self evaluation that can give you an idea of the range of what "fluent" can mean (http://www.govtilr.org/Skills/readingassessment.pdf).
WHAT LEVEL OF PROFICIENCY DO YOU WANT
The next question you have to answer is what proficiency level are you aiming for.
Most of the resources you listed are fairly good at getting a learner to a low level of proficiency (CEFRL A2, ACTFL Novice High, or ILR 1). Just try one or two and find the one you like doing (I am a fan of Duolingo, but ymmv). This level is roughly "survival mode" language (e.g., basic introductions, basic getting around and doing things, short and simple small talk). If your goals are higher than that, then then the process is less transparent, but it mostly involves working with authentic native materials (texts, videos, audios, etc.) and learning through interaction with those materials. Note that it is almost impossible to get beyond a very low level of proficiency with books alone -- the scope of language that would need to be covered gets too large too quickly. As your proficiency level increases, language learning texts become reference sources rather than primary sources of learning.
The steps of fluency roughly look something like this (using ILR scale for simplicity):
- Memorized words and phrases (ILR 0+).
- Short, simple sentences (ILR 1). Many/most Americans I know consider this to be "fluent".
- Basic paragraphs (ILR 2).
- Extended prose (ILR 3).
Most of the suggestions I see in this thread focus on ILR 0+ and ILR 1. There is an entire world of language and language learning beyond that. Note that I stopped at ILR 3 -- that's the level at which a person can fully function at a professional level in most contexts. Day-to-day life is largely conducted at the ILR 1+/2 level.
WHAT SKILL
How do you want to use the language? The four skills are reading, listening, speaking, and writing. The first two are receptive skills that develop faster than their productive skill counterpart. Note that materials that are really good for developing one skill (e.g., reading) might be much less effective or even slightly counterproductive for learning another skill (e.g., speaking). That said, it is usually good to develop all skills at least somewhat while focusing on the skills you are most interested in (i.e., if you want to read, don't just read -- learning some speaking and listening will help the development of your reading).
HOW MUCH TIME
Another question is how much time do you have to dedicate to learning the language. Some languages are more linguistically distant from your native language than others, and the more distant languages take longer to learn. Here is a scale used by FSI with languages and hours of instruction needed to get to ILR 3 in one skill (usu. in speaking):
http://www.effectivelanguagelearning.com/language-guide/lang...
Note that the range of time required is large -- 600 hours in 6 months for Spanish or French, but 2200 hours in ~20 months for Arabic, Chinese, Japanese, or Korean. To put that in perspective, when a talented learner of Spanish is functioning at a full professional level, an Arabic learner who started at the same time will typically be functioning at a touristy sentence level.
BASICS OF PROCESS
Maybe this is a tl;dr. I am not sure that it makes sense without the above context. Note that at any time, traveling to or living in a place where the language is spoken will help tremendously. Also note that having a native informant can be very useful -- italki is a great resource for native informants.
1. Assuming you want to learn a relatively commonly taught language (e.g., something like Spanish or Korean rather than something like Xhosa or Igbo), pick any learning source that you like and stick with that. You will learn the sounds and script of the language as well as memorize basic words and phrases. You will eventually be able to create short, simple sentences that may or may not sound native-like. This is about ACTFL Novice or ILR 0+ or 1 level.
2. Start looking at level-appropriate native texts, and use learning texts as references rather than primary sources. Lower-level texts might be things like ads, announcements, or parts/clips from videos that cover casual conversation. Higher-level texts might be newspapers, non-fiction books, most general interest TV shows (i.e., not ones on opinionated and/or abstract topics like politics or religion). Flashcards can be useful (esp. for specialized vocabulary in a field you are interested in), but you will want to move away from flashcards and memorization gradually. You will need to immerse yourself in the language as much as possible to approach full functionality. This does not require you to be in a place where the language is spoken, but that usually helps a lot. This will get you to the ACTFL Intermediate or ILR 2 level.
3. To go beyond step 2, you will largely need to start functioning like a native. Your day-to-day socializing and media consumption will be almost entirely in the target language. The reference texts you typically use will be the ones that are written for native speakers of the language you are learning (e.g., a Japanese-Japanese dictionary). This is ACTFL Advanced or ILR 3 level.