Everyone has an instinct for this and the instinct is roughly "capitalise the important words". It works until someone asks about with, or through, or a hyphen, and then it turns out that four widely used style guides answer differently and each is internally consistent.
What all four agree on
The common ground is larger than the disputes, and getting it right covers most titles.
Capitalise nouns, pronouns, verbs, adjectives and adverbs — always, regardless of length. This is where Is, Be, It and My live, and it is where most mistakes are: people lowercase them because they are short, and shortness is not a rule in any of the four.
Lowercase the articles — a, an, the — and the coordinating conjunctions: and, but, or, nor, for, so, yet.
Always capitalise the first word whatever it is, and the last word too in three of the four.
Where they split
Prepositions. This is the real dividing line.
AP uses length: any word of four letters or more is capitalised, so it writes With, From, Over, Into. Chicago uses function: prepositions stay lower case however long they are, so it writes with, from, over, between, throughout.
Which means the same title comes out differently and both are correct: A Walk Through the Woods in AP, A Walk through the Woods in Chicago. MLA follows Chicago here. APA uses a length rule like AP but sets the threshold at four letters as well.
To. Lower case as an infinitive marker — How to Cook — in all four. Chicago keeps it lower case as a preposition too. AP capitalises nothing of two letters, so it agrees by a different route.
Subordinating conjunctions — if, because, although, as. Chicago and MLA capitalise them, because they are not coordinating conjunctions. AP capitalises the long ones by its length rule and leaves if and as lower case. This is the source of a great deal of quiet inconsistency in headlines.
The two that catch everybody
Hyphenated words. All the major guides capitalise both parts — Well-Known, Long-Term, State-of-the-Art with the small words inside following the usual rules. Chicago makes an exception for a prefix that cannot stand alone: Re-entry, not Re-Entry.
Automatic converters usually get this wrong, capitalising the first part and leaving the second, because they split on spaces and a hyphenated compound is one token.
The last word. Chicago, MLA and APA capitalise it whatever it is, so a title ending in a preposition keeps it capitalised: What Are You Waiting For. AP does not write the rule down but follows it.
Sentence case, and why it is winning
Sentence case capitalises the first word and proper nouns, and nothing else. It is standard in British publishing, in most modern software interfaces, and increasingly in documentation.
The argument for it is not aesthetic. Title case requires a decision about every word and produces inconsistency across a team, because everyone's instinct is slightly different. Sentence case has no decisions in it. For interface text, where the same phrase appears in twenty places written by six people, that is worth more than tradition.
Title case remains correct for American publishing, for book and article titles, and in academic reference lists — where APA in particular uses title case in the text and sentence case in the reference list, which is a rule worth reading twice.
Converting reliably
A case converter does the mechanical part and you check the rest, because the parts a converter cannot know are exactly the parts the guides care about: whether a word is a preposition or an adverb in this sentence, whether a hyphenated compound has a prefix, whether a name is meant to be lower case.
Three things always need a human pass. Proper nouns with internal capitals — iPhone, eBay, LaTeX — which every converter flattens. Acronyms, which a lower-casing pass will destroy. And brand names that are deliberately lower case.
The practical workflow is to convert, then read the result once. That is faster than capitalising by hand and safer than trusting the output.
The short version
Capitalise nouns, verbs, adjectives, adverbs and pronouns however short. Lowercase a, an, the, and the seven coordinating conjunctions. Capitalise the first and last words regardless. Then pick a rule for prepositions — length for AP, function for Chicago — and apply the same one every time.
Consistency across your own pages matters more than which guide you picked. Nobody will notice that you follow Chicago. Everybody notices that two headings on the same page disagree.