Anyone learning German, and not previously having studied a language with a case system, shouldn't have too much trouble with declension of most German nouns, with the exception of certain masculine ones. But adjective declension is something else. Looking at a table such as this, one can see there are 48 different entries for those containing adjectives, although many endings have multiple entries, especially -en. Different courses, both written and online, suggest different ways to learn them, some by ignoring the endings for the genitive so as to simply the matter. I find it easier to use a chart like this that contains examples, rather than a chart of bare endings. What I especially like about this chart is that one gets both the declension of articles as well as adjectives, and some of the noun endings as well (excluding certain masculine nouns which vary in the dative).
Those are the things which I can't do in German. My German is good I think, I can understand all and say all, but I mess up the case declensions. Basically, I ignore them, and say the one which seem correct, but for sure I make a mess of this declensions Sorry, German, but you better make yourself simpler!!!!!!
I'm sorry, but we're going to have to insist that you remove German from your list of languages until you have passed a timed declension test with a score of 90% or better . Why not just use that chart and take one area at a time, like the masculine accusative, and practice being conscious of it when you hear or read it, and then practice it in any speaking or writing you might do. If you methodically work through it a little at at time you will get it down before you know it.
Oh no YIKES I would not have a score of 90% probably 9% LOL I can't do a test but we can write more in German here instead It's a great idea!!! I didn't think of this, to do only one section. thank you very much for this idea