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).