The first step should be to acquire a storage cabinet, preferably made of plastic as these are lighter in weight, making the darkroom more portable. The size of the cabinet depends on your requirement and the portability which you need. You should also acquire wheels and attach them under the cabinet.
Once you have bought the cabinet, install a collapsible table which can be used as a counter. The base or the surface of the cabinet should be made of a material which can be cleaned without any trouble.
The area where you will be working should be big enough to accommodate at least three processing trays.
The provision of running water near your work area is crucial and for this reason you can set up the cabinet somewhere near your kitchen or bathroom. If you want to place your darkroom in a hall or a bigger room it is better that you use a location with only one door and without too many windows as this will make light proofing a lot easier.
You now need to fix any leaks in the room in order to make it light proof. You can choose to work at night but it is still a good idea to test your facility for any leaks beforehand. You can do this by turning the lights off and then observing the whole room.
You need to cover the gaps in the doors and windows with black tape or a blackout sheet. You can also install a large umbrella or a cloth outside the darkroom to keep the light out.
Once all the leaks are secured, you can begin using the cabinet as your portable darkroom.