After about a week of practice from ground-zero, this has been my best result, which I am not super happy with. (My other underpaintings are significantly worse and are crumbled up in the trashcan so I cannot share them).
I'm using Old Holland Burnt Umber, with a mixture of Gamsol and Walnut oil (about a 2:1 ratio), and am mixing them together in a small glass container which I use for the entire underpainting, wiping off the excess as needed.
I am using a size 3 point round hog bristle brush on a 9x12 acrylic primed canvas paper to apply the paint, but it's impossible to apply any level of detail or value without smearing everything together into a cohesive blotch.
What am I doing wrong here? How are other people able to apply details and varying values of light and dark without causing a huge mess?
I've since bought some smaller badger hair brushes and Liquin, in the hopes that it might help with creating better value & detail