I linked to some hopefully helpful resources / examples in this recent thread (though the focus there is mostly on going in the other direction, from Observable to standalone JS).
Early on, when I was learning both D3 and Observable, I wrote this notebook, which tries to explain the steps I took in converting a block to Observable:
You can find a bunch of other examples where folks converted ordinary JS / HTML to Observable notebooks if you search for notebooks containing “bl.ocks.org”.