Should I write Deutschpräsentation or Deutsch Präsentation? Are both correct? Is one simply more used than the other? Or does the first word not even exist? Please be specific in your answer so I can apply this to other words as well. For example, for your answer provided, would it also apply to all other subject matters? (Geschichte, Französisch, Englisch, etc.).
There is a clear difference between the two. I very, very strongly suspect that you want to use the first.
Compound nouns in German are formed by connecting two nouns without a space. This is written in the official spelling rules of German, § 37:
Substantive, Adjektive, Verbstämme, Pronomen oder Partikeln können mit Substantiven Zusammensetzungen bilden. Man schreibt sie ebenso wie mehrteilige Substantivierungen zusammen.
Nouns, adjectives, verb stems, pronouns or particles can form compounds with nouns. They are written in one word just like multi-part nominalisations.
There is only one further rule which allows for deviation as long as the subjects in question are composed of proper words (no symbols, no digits, no abbreviations) and non proper nouns, namely § 45:
Man kann einen Bindestrich setzen zur Hervorhebung einzelner Bestandteile, zur Gliederung unübersichtlicher Zusammensetzungen, zur Vermeidung von Missverständnissen oder beim Zusammentreffen von drei gleichen Buchstaben.
A hyphen may be typeset to emphasise single components, to structure unclear compounds, to prevent misunderstandings or if three identical letters would meet up.
(I emphasised the hyphen. It is always a hyphen, never a space.)
Thus, under the very likely assumption that you were attempting to form a compound noun (history presentation), the only accepted spelling is Deutschpräsentation — unless you need to clarify or emphasise, in which case Deutsch-Präsentation (but nothing else!) would be acceptable. (Hyphenation at the end of a line is a separate matter, but in a nutshell if you hyphenate at the end of a line the word is continued in lowercase letters; distinguishing hyphenation hyphens from explicit hyphens.)
Now what about Deutsch Präsentation? It bears repeating that in over 99 % of cases this spelling is incorrect. It creates two distinct semantic entities whose sole remaining grammatical relationship is being in the same sentence. With a lot of strain, I can contrive a highly fictional example wherein the two words would be ‘correct’:
Ja, das passiert immer, wenn die Deutsch Präsentation spielen will.
I put ‘correct’ between inverted commas because it is really nothing more than grammatical correctness; the sentence is extremely poor semantically. It may be understood to express that something always happens if a person called Deutsch wants to play Präsentation — whatever the meaning of that is. Note that while it is hidden, the two words are different grammatical entities and in different cases.
Therefore, your desired simple rule:
If you want to create a compound noun, write it as a single word with no space.
Among these two options, Deutschpräsentation should be used.
From context it might be clear, but on its own I would have difficulty understanding what a Deutschpräsentation is. Is it a presentation about the German language, is it a presentation about Physics given in German,...?
To avoid misunderstanding, I'd be more specific and write something:
eine Präsentation im Fach/Kurs ...
eine Präsentation in deutscher Sprache....