If you want esoteric, academic discussions of the mechanisms by which people learn their second language, or the quantization of various hypotheses compared to each other, I suggest you go here.
If you want to study the practical pathway for picking up a second or foreign language, we can do that, and when you've finished here, you can find more on this topic to study, but I agree with Northwest Regional Educational Laboratory's stages, (more for teachers than students) as these fit quite well with the developmental steps I've encountered in my real-world teaching and learning of second languages:
The Silent/Receptive or Preproduction Stage: (I learned this as The Formatting Stage ) This can be a period as long as several months or as short as a few hours. The learner is 'formatting' the language, more or less consciously, not yet ready to produce words or phrases, but observing, soaking up and choosing to be immersed in and mindful of a wide variety of situations, even if the learner doesn't PRODUCE any of the observed vocalizations. The learner may accurately hear and recognize, in this stage, several dozen to several hundred words or phrases in the target language. This stage has a conscious component, but is mostly happening at the subconscious level.
The Early Production Stage: ( I characterized this as Crawling, or Stumbling ) Students here can stumble along or crawl about, giving 1 or 2-word answers in response to questions that they comfortably recognize. This is a very conscious process, often involving an active 2nd-Lang to Native to Meaning to 2nd-Lang response-retrieval or recall type process. It is clumsy, the learner feels clumsy and often embarrassingly inadequate, as is normal, for the human's conscious mind is NOT up to the task of speaking any more fluently than this. Learners live here for a few weeks to a few months, with short, yes-no answers and clumsy, labored production.
The Speech Emergence Stage: ( I'm Walking! ) Walking in the learner's newly-acquired language allows the learner to meet and respond to a wide range of habitual, daily chores and routines, now with only occasional conscious-mind assistance or interference. The speech production process is beginning to be transferred to the subconscious mind, with the conscious mind doing little more than choosing subjects to discuss or occasionally dredging up a word or a workaround for an as-yet-to-be-learned word. Learners move through this valley in a few months to a year-plus. Students can be here with as few as 2,000 well-chosen words, but 3,000 to 5,000 is more common. This stage is sometimes marked by the learner's first dream in the second language.
The Intermediate Language Proficiency Stage: ( I'm loping along now, skipping and jumping! ) This is where real reward starts kicking in, with the learner engaging in real give-and-take, dialog and question-and-answer type repartee. An extensive vocabulary has seeped into the subconscious, and is at the learner's beck and call.
The Advanced Language Proficiency Stage: ( Look, Ma! No hands!) If the learner has been using the acquired second language on a frequent, daily basis for practical, real-world activities and information-gathering, this stage can be achieved in one or two more years.
It's those first three stages that can be most demanding, so in order to minimize hassle and optimize learning, review "Second Language" series by this Author.