Get a few lengths of bonsai wire, in the smaller sizes.
The difficulty is not getting them to go up, but to branch out sideways at a low height as opposed to a tall column. It does no good if your first horizontal branches are 50 scale feet off the ground, the tree will never be a miniature that looks right next to your locos.
Take a look at the sites that do this, and just look at how bonsai turn out, you will see the structure you want.
Again, there is so much out there on how to do this, there’s no reason for me to even attempt to replicate that information here. My site is a collection of plants that are good candidate for what seems to be to be realistic trees next to my layout.