GraphViz – How to have a subgraph be left-to-right when main graph is top-to-bottom?

The trick to get the graph you described is to use two subgraphs and link from one to the other. The invisible edges in “details” are what keep the notes aligned.

digraph {
    rankdir="LR";

    subgraph steps {
        rank="same";
        "Step1" -> "Step2" -> "Step3";
    }

    subgraph details {
        rank="same";
        edge[style="invisible",dir="none"];
        "note1" -> "note2" -> "note3" -> "note4";
    }

    "Step2" -> "note1";
    "Step2" -> "note2";
    "Step2" -> "note3";
    "Step2" -> "note4";
}

The result is: