How to master a foreign language

STEP 1: Think of a language as a way of expressing meaning. Each language chooses different sounds and arrangements of sounds, to express meaning.
STEP 2: Try to "forget" your native language's grammar as much as you can while you're learning the new language. This will prevent "interference" from your native language.
STEP 3: Identify how your native language and the new language do things differently and keep these in mind when speaking the foreign language.
STEP 4: Review early and often.
STEP 5: Find a tutor to give you feedback on what you're doing correctly and incorrectly.
STEP 6: Keep lists of new vocabulary and sentence constructions. Review as much as you can each day.
STEP 7: Visit a place where the language you're studying is a native tongue, if possible. Speak your own native tongue as little as possible while you're there.

Tips & Warnings
Get a good bilingual dictionary. It'll go a long way to helping you build new vocabulary.

If you can't visit a place full of native speakers, then watch movies and TV programs in the native language. Read newspapers, too, and learn songs.

Don't be discouraged when the words don't flow. The only way to really master the grammar is to speak the language a lot. Develop a thick skin.

If you're afraid you'll sound like a fool, you won't ever learn how to speak correctly.