Extracted from the current manuscript of the book, to the right you'll find the meat of how my preferred method for learning grammar works, in convenient flow chart format.
As with learning any piece of knowledge, you'll learn a grammar rule best through spaced repetitions. As such, through much trial and much (much, much, much...) error, I've found that combining a wide variety of repetitions works best. Although the repetitions do not have any systematic spacing based on a forgetting curve as spaced-repetition systems are supposed to, there should be enough repetitions here to get the rules in your head.
Let's take a walk through that flow chart, after the jump.
First, let's look at the big picture. The first three steps get you started in developing an understanding of the grammar. These steps are front-loaded in the process and can typically be taken care of in a few weeks (if you're going to the language zone, they are also good things to do before you get there). The next two steps move onto exposure, which is by far where you'll be spending most of your time. Finally, the last four steps allow you to refine your grammar to cover things that haven't already clicked, but you only need to proceed to those steps as necessary. The flowchart then always turns you back to exposure.
It's the time you spend getting repetitions of grammar. When you're just getting started, most of those repetitions—and thus your grammar time—will come from you reading and working through the grammar, with some exposure to the same from the other things you're doing in the language. From there on, most of your reps will come from exposure, and you'll also spend a gradually decreasing amount of time refining your grammar, until finally you're spending virtually no time at all focusing solely on grammar.
That's the big picture. Now let's run through each of the steps.
Reading through your grammar is just what it sounds like. Get the grammar in front of you and read through it. This should be a relatively quick read-through; don't get hung up if you don't get something right away. Try and get an idea of the rule and move on. You shouldn't spend more than, say, 5-10 hours doing this (and you can probably get away with doing even less). This read-through will give you your first repetition—a relatively weak reading rep—of all the grammar rules.
Then we get to outlining. What exactly do I mean by outlining? Taking the grammar you see in front of you (another reading rep of the grammar rules), trying to make sense of it (an analysis rep, which will cause the rules to more readily stick in your head than mere reading reps), and then outputting it into a Word document or the like (a writing rep) in a way that makes sense to you (be sure to include plenty of tables, charts, etc.). The document you produce should be a raw and bare explanation of the rules, using example sentences and the like only when absolutely necessary.
The outlining step will typically take some time, but you should be able to get through it in several weeks (if you're studying the language full time, make that 2, maybe 3, weeks, assuming of course that you're doing other stuff as well; if you dive into an outline non-stop, I'll bet you can kill it off in less than a week, although I've always preferred mixing things up a bit more than that). If it's taking you a lot longer than that, you should reduce the level of detail you're putting into the outline. For anything that you ponder for, say, 10 minutes, and still don't get, mark it as a question in the outline and come back to it after you've covered everything else.
If you still don't get it after your second pass at it, it's time to ask a native speaker or a language-leanring forum. I often throw tricky questions to native speakers on Lang-8, and it's also common practice on italki. Some good forums include How to Learn Any Language and WordReference. These won't be able to do everything in every language, but there are of course plenty more forums to be had through a few simple searches. And feel free to ask any native speakers that might be nearby, as well. Once you get an answer from any of these sources, update your outline accordingly.
That takes us through getting started, and now it's on to exposure. This just means reading, listening, writing, and speaking the language. (Here's a little process for doing all four of those things using free, online tools.) This will get you exposure to the grammar in use, both passively (reading, listening) and actively (writing, speaking). While this requires little explanation, it will be by far the place where you get most of your grammar reps, with the goal of it ultimately being pretty much the only place where you get grammar reps.
Finally, we come to refining your grammar knowledge. If there's something you see or hear and don't understand, or if there's something you keep screwing up in writing or speaking, it either means that the grammar rule is new to you or that you've forgotten it. In either case, the refinement process will help you get the additional reps you need to burn the rule into your brain.
The first step is to simply review the problematic grammar in your outline (a reading rep). If you've already got the correct rule in there, review and get back to getting exposure. This will typically be the solution when you've merely forgotten the rule and just need a refresher.
On the other hand, if the rule was never in your outline, or you didn't really get it right the first time, then it's time to edit the outline. Add or fix the rule, as necessary, and get back to exposure.
If you don't get the rule, or think you've got it but still seem to be getting it wrong, it's probably time to turn to others for help. Just like above, native speakers and forum participants should be able to answer your questions.
As an absolute last resort, you can actually memorize problematic grammar. This means making grammar items in your spaced-repetition system. This should generally be unnecessary, but if you've found that you keep going through the refinement cycle without really getting the rule, this can help you finally nail it down.
And that's it. At the end of all this, the process should lead you to getting just about all of your grammar reps from exposure, with a very occasional dive into the refinement process.