All there is to it is finding a suitable spot and fabbing longer cables, which will likely be necessary.
The solenoids should be mounted as high in the engine compartment as possible. That's one advantage of locating them under the hood, as you can usually find a higher spot than on the front bumper.
Try to avoid it being in direct line with your fan, as it will tend to splash water on them when fording.
Don't skimp on the size of the cables and if they are going to be more than 50% longer, then I'd bump up the size of the wire one gauge from whatever Slee used in the present location.
Be sure to provide plenty of electrical and rubbing insulation where the cables go through the front clip to get inside the engine compartment.
I have a wired controller and can leave the hood ajar on our 80 without pinching the controller cable where our solenoid pack is located under the hood. Also be careful on placement of the controller's plug-in, as it shouldn't be too close to the fan that you might get a finger into it getting plugged in.
Another option could be a wireless controller or an in-cab control, although I personally would recommend retaining some sort of remote controller even with in-cab controls. I've seen circumstances where you didn't really want to be in the truck when winching.
I relocated the solenoid for my Warn 9.5. Fabbed a simple "L" bracket to a higher, dryer and easier to access location; installed it to the DS fender; ran longer electrical cables; integrated Slee's in-cab controller circuit box so I could, if I want/need to control the winch from in the cab (I can also use the standard Warn winch controller from the solenoid too).
Working on my setup now. I have extended the cables and fabricated this mount from some spare nissan pathfinder bumper brackets. Hope it helps someone. I was able to bolt it to 2 already threaded bolt holes in the body. Will post more pics when i finish. (Fyi this is for a Warn M8000)