How do I enroll in a German preparatory college (Studienkolleg) at a public university?

There is no separate "German preparatory faculty" in Germany; what people have in mind is a public Studienkolleg, and how you apply depends on the institution and the Land. A Studienkolleg is meant for applicants whose school certificate does not give direct university access, so the idea that you need a university entrance qualification (HZB) first is wrong.

#Where to apply (examples)

• Through uni-assist: Frankfurt (Goethe-Universität).
• To the university you want to study at: the university Studienkollegs in Bavaria; according to Heidelberg, you first apply to a university in Baden-Württemberg and are then invited to the entrance exam.
• Directly to the Studienkolleg: Hamburg and the Bavarian FH-Studienkollegs.

uni-assist itself says it is not automatically responsible for Studienkolleg applications, even at member universities, so check your target institution's page.

#German requirement

You must prove German before admission, at a level the institution sets. Examples: Frankfurt B1 at application (at least B2 for a realistic chance), Hamburg B2, TU Berlin an entrance test at B2 level, Bavaria at least B1+/B2 in the entrance exam. An entrance test usually follows; Hamburg has none and uses a points system instead.

#Deadlines

Studienkolleg deadlines are often earlier than degree-programme deadlines, and there is no guaranteed place.

Source: uni-assist – Studienkolleg · Studienkolleg guide