It is comparatively easy, although in my case it was also easier because I already owned the vehicles.
You do need an experienced customs agent to clear the vehicle unless you have a lot more time than money. Their services aren't that expensive anyway, but they'll make sure that you fill in the correct forms for EPA and NTHSA exemption forms, get the vehicle through customs and all that so you have the correct paperwork. I didn't have to pay any duties but there's a chance you will have to, but again that's something the customs agent will handle, you just have to stump up the monies.
Oh, and make sure that for any car you buy in Japan, you get the correct certificates of deregistration. IIRC there might be issues getting the car out of the country if it hasn't been deregistered correctly and there might be issues registering it here.
If you have the right paperwork, registering the vehicle shouldn't be that hard - in my case, the NV DMV simply treated it as an out-of-state purchase so I had to get a VIN inspection, hand in all the paperwork including the customs stuff and it got titled and registered without a problem. However, in my case the title was branded as a non-US market vehicle and none of the regular inscos would touch the bikes. Hagerty had no problem with that but State Farm, Allstate & friends said "no".
I personally have never bought a car from Japan, but friends of mine in the UK had a side business doing that. You'll need a buyers agent in Japan who can purchase the car at an auction, get it to the docks and arrange shipping. I have no recommendations for them but it shouldn't be too hard to find one.
As to shipping, most cars will come over RoRo and you'll have to arrange for shipping from the docks. Storage at the docks is expensive so make sure it doesn't hang around there for too long (not to mention that in my friend's experience, parts that were attached to the car when it left Japan might go walkies at the dock - happened more than once).
Unless you have unfettered access to a loading dock and a way to get a car from the loading dock down to ground level do not ship a car in a container to your place. You can probably get the container set down at the dock and unload the car there, but if it shows up at your place on a truck and you have no loading dock, you have a bit of a problem. That said, container shipping is a lot more expensive than RoRo, so unless you're importing six figure tuner cars, I'd go RoRo.