It depends on how you're trying to apply the physics. If you're just trying to get parts to bounce along with the body, (armor on the ass to shake when you walk or whatever) I can give a basic tutorial.
Open the armor in Outfit Studio (with HDT SAMBody) and set it as reference.
Select all the armor pieces and copy bone weights at the default settings, then file > export > to nif with reference. Do this for both the _0 and _1 and that will give you a very basic HDT armor. The results however will heavily depend on how many vertices/polygons the armor has, whether there's any hanging cloth like the above, how close the armor/clothing is to the body, and if the mesh was weighted to the proper bones.
A low poly mesh (like most vanilla clothing and some vanilla armor) will be very jarring and spiky with HDT physics applied (which I learned the hard way).
With hanging cloth, you'll have to mess with skirt bones and that's another can of worms entirely. It's a much longer process from what I've gathered, and I've never found a comprehensive tutorial. It's why there's only a handful of HDT cloth mods out there and most armor mods with flowing cloth get permission to include the HDT cloth in theirs. So usually for armors with a skirt I either use the mask tool on it to leave the skirt bones alone and just deal with the stiffness, or just weight them to the thigh/legs and deal with any awkward movements.
The last two factors are connected. If a piece of armor is too far away from the body you can either manually weight it to a bone (which gets better results but takes more experience to do properly), or you can just increase the search radius (which is the faster/simpler way but with more possibility for error). You can end up with armor on the thigh being weighted to your forearm bone and weird things like that.