I've got a bunch of these, both
little volunteers I find in my yard (coastal variety) and ones that I
acquired from ArborDay . They can be treated like a spruce/fir as cut back to a bud. Any stem (internode) without a bud the end will become a dead stem, so one must cut back to a node or a bud. I continue to do this in late summer, after the new foliage has all hardened. With spruce this seems to strongly stimulate back budding, even on the trunk of the spruce, but it doesn't seem to work that way on pseudotsuga. Buds sometimes show up on leafless wood, but for reasons I still cannot decipher. They just do their thing with the only predictable budding being the abundance of budding on the dominant leader. I have read in forestry papers that the crowns of forest Douglas firs spontaneously renew (bud) after 8 or so years which might mean back budding is simply age related, but I know that is not exactly true either.
Maybe
@wireme will chime in. He's got several Douglas fir yamadori and has a better grasp of this stuff that find to be interesting but which continues to confound and elude my understanding.
Otherwise, it is just the same ole 'make a tree' problem
. Wire it and bend the hell out of it it you want, they are very flexible and readily set up / lignify in position. Let the main trunk be a sacrificial leader, then cut it off and choose a low branch to be the next trunk segment, etc. etc. None of mine are bonsai and I'm not at all confident they ever will - time will tell. I enjoy fiddling.