Pretty fluent in English and Russian, my German and Serbian usually take about 2 weeks being in the country to come fully back to me. My French is enough to buy stuff and read the paper, but not to have a meaningful conversation.
I would start re-learning the basics first, in the following order:
1) The verbs Haben/Sein
2) Expressions of time
3) EACH DAY, practice the following table until you've memorized it by heart:
https://i.pinimg.com/originals/8f/89/64/8f8964eded93c38a89954d0fe9638815.pngIf you learn this table early, when you start learning to apply grammatical cases you'll pick it up much quicker. It also allows you to puzzle together sentences like a native (oh, here it's
den, so it's either a masculine or a plural word)
4) "where", "where to" and "where from"
5) "to", "at", "in", "on", "from"
6) the
mein sein dein ihr unser table
7) "which", "that" , "who"
8) negatives (no one, nothing, not yet, etc...)
9)Adverbs of quantities and degrees
10) Adverbs of position and direction
11) "how", "so", "also"
12) And only after all that, you start with the grammatical stuff of present/past/future tenses, and the application of the grammatical cases.
Whilst continuing practicing grammar, continue on with:
13) sein, in combination with sitting, standing, lying, to put
14) sein in a negative context (i have not, you are not, she isn't, etc..)
15) getting married, divorced, being born
16) dying, being ill, hurting, loving, liking
17) verbs of motion (fahren, gehen, etc..)
18) spelling and grammar rules (when to capitalize, when to use a point in a date, etc...) this should already be familiar from the "time" lesson
Reading german children's books and watching german children TV is a great way to practice the language, although i personally prefer watching commercials/infomercials.and reading about subjects that i already know like the back of my hand.
Also, the best way for me to learn a language, besides living there, is to ONLY watch news/tv/movies from that country or dubbed in that language. If you can receive/stream ARD, ZDF, WDR or RTL, that would be outstanding help, but there are plenty of german TV shows and movies worth watching (the Heimat 1/2/3 Chronik and Goodbye Lenin, are among my favorites)
Replacing the labels on my bottles with the ones from the country and before using something is also a good one, first repeating what's on the package 3 times (this is not for everyone, but I have plenty of time
and OCD, so it works for me)
@Alicatt: it's not you, some flemish dialect have more to do with Klingon than with Dutch. In The Netherlands, we actually had the "Jambers Quiz" going on for a couple of years, where the objective of the contestants was to translate a belgian dialect sentence without subtitles, into proper Dutch. It was hilarious!