What is a good learning scheme to learn a language?
For instance french or German.
May not be the best, but this is what I do:
(1) Duolingo (obviously, or I wouldn't be here);
(2) start drilling early on verb conjugations, starting with a few regular verbs but progressing to the common irregular verbs. I like to use a flash card app with spaced repetition. Getting conjugations down cold makes things easier later.
(3) Work through some grammar work books.
(4) Flashcard app for vocabulary.
(5) Identify and give special attention to problem areas for the given language. For Spanish, proper use of the various verb forms. For German, der/die/das plus the case structure. Every language has something you will find hard.
(7) Get a pen pal, or a conversation partner, or someone you can practice with and give you feedback.
You mean with Duolingo? Depends on your learning style and how much time you can invest, but this is the way I like to do it:
I learn one to three new skills per day maximum (i.e. the colorful circles in my language tree). Each skill usually consists of somewhere between two to ten individual lessions, and I limit myself to no more than 10 new lessions per day. The rest of the day, I strengthen the heck out of the skills I have already finished. I usually rack up 200-300 XP per day altogether.
In addition to that, if I have the time, I work a bit on my reading skills with bliubliu.com (I also tried readlang.com, but I prefer bliubliu so far), or by reading the news or a book if my skills are good enough already.
I get bored easily, so I like to switch between languages now and then. For the sake of learning efficiency (and my sanity), I have settled on a "one week, one language" policy - i.e. when I study a language, I do so exclusively and at least for one week.
Have fun! :-)