One of these days, we were going to have to get around to gender agreement in quantifier pronouns, and it might as well be today! When you say the phrase "one of these...", the word "one" is an indefinite pronoun called a "quantifier pronoun." In German, if you refer to two or more of something as a quantifier pronoun, you don't need to worry about the gender of the noun that it's referring to, you just say the number zwei, drei, vier, or however many you mean to say. But if you are referring to just one thing, then the quantifier pronoun "ein" has to match the gender of the noun to which it's referring.
There are two common ways to express the phrase "one of these..." in German. In the first way, the quantifier pronoun ein is followed by the dative preposition von and the noun which is being referred to:
Wir nehmen eins von diesen Modalverben,
We'll take one of these modal verbsPlay Caption
Und dann nehmen wir eine von diesen Präpositionen und dann wissen wir, es ist Dativ.
And then we take one of these prepositions and then we know it is dative.
Caption 12, Deutschkurs in Tübingen WechselpräpositionenPlay Caption
Such dir eine von diesen vielen Banken aus.
Select one of these many banks.Play Caption
In the first example above, the quantifier pronoun ein is referring to the neuter noun das Modalverb, thus it is rendered neuter as eins. The same logic proceeds with the following examples: the feminine nouns die Präposition and die Bank require the feminine quantifier pronoun eine. Thus, if you used a masculine noun such as der Mensch, you'd say einer von diesen Menschen...
The second way to express the phrase "one of these..." in German is using the quantifier pronoun ein followed by the genitive case of the article:
Und eine dieser Sachen war, äh, und ist auch noch, Plastikverpackung.
And one of these things was, uh, and still is, plastic packaging.
Captions 59-60, TEDx Der Supermarkt der Zukunft - Part 1Play Caption
Hinter einer dieser Türen steht ein geiles, neues Auto.
Behind one of these doors is an awesome new car.
Caption 15, Mathematik Das Ziegenproblem/Monty-Hall-ProblemPlay Caption
The first example above is very straightforward, using eine for the feminine noun die Sache. The second example follows the same construct, but since the quantifier noun is preceded by the dative preposition hinter, the feminine quantifier pronoun eine must be rendered dative as einer to agree with both the dative preposition hinter and the feminine noun die Tür.
It's a bit complicated in theory to always have to think ahead to the gender of the noun you are going to modify, but with a little practice it's something you'll soon get used to!
Go to Yabla German to find more examples of "one of these." You may also find a tandem partner that you can work with to make up some phrases using "one of these" in English, and then try making your own translations into German, cross-checking each other's work. Be sure and translate using both the dative and genitive versions that we learned about above!