Most investigations go wrong before any reading starts: the question is too wide to say what to open. Narrow the question and watch which sources it calls for, then how each one is acknowledged.
A footnote sits at the point in your text where a claim is made, and it answers one question: where did this come from? Number the marker in the text, then give the full reference the first time: creator, title, place and publisher, year, and the page or item. Every later note to the same source can be shorter, but it follows the same pattern as every other short note. Consistency is the whole point: a reader should be able to find the exact page or poster you used. Footnotes also carry minor clarifications that would clutter the text, such as a translation of a German title. In any historical investigation, a claim without a footnote is a claim the reader cannot check.