I don't have a darkroom, but just use a changing bag to load and unload the holders and to load the tanks. Processing is all done in daylight tanks.

Up till now, I have been processing my 9x12cm and 4x5" sheet film in my 120 tank. For a single sheet, I just bend it emulsion side in, put in the centre column and put the lid on. Process as per normal using inversion, ensuring you have enough chemicals to cover the sheet. You can if you want just roll the tank around if you want to use less chemistry, but since I use Rodinal or PC-TEA at 1+50 or 1+100, volume is not really an issue.

If I have more than one sheet to do, I use the taco method. Bend the sheet, emulsion side in and slip an elastic band over. The elastic band should be tight enough so that the the opposite edges of the sheet almost, but not quite, touch (a gap of a little finger is perfect). I can fit four tacos in my 120 tank. Develop using inversion as per roll film.

I've just received a Paterson Orbital though. This will process up to four sheets of 4x5 or two 8x10s. It looks like a tray with a light proof/trapped lid. Uses around 250ml of chemicals as far as I can tell and have read. I have the manual base, but you can also get a motorised base. I'll have to give it a spin this weekend.