Here ya go Ezy. Everything you wanted to know about flat fields, darks etc from one of the masters of the game,
Eddie Trimarchi
I usually take flat fields at the end of the night after I finish imaging, or the next day even. The important thing is to keep your imaging train in one unchanged piece. That means don't move anything! Take your scope off the mount with your DSI and all the imaging bits and pieces still attached and carry them inside (or leave everything on the mount if that's how you normally move it about). The important thing is to keep your imaging train in one unchanged piece. Get the idea

Don't move the camera or reducers or focuser or anything.
I'm fortunate in that I have a very bright light source reflecting of a very pale coloured wall. I place my scope as close to the wall as I can without any sort of shadowing, about half way up, then I put a piece of fairly transparent white muslin or cheesecloth over the aperture and then start imaging. With the DSLR I take a series of shots, maybe 5 @ 1/ 4 sec, 5 @ 1/2 sec all the way up to 1 or 2 sec to get the right exposure. It's not difficult not time consuming, I just need to make sure my illumination is even over the area being imaged. By getting the scope close to the wall it reduces the light illumination gradient from top to bottom of the wall. I'm usually withing 300-400 mm of the wall.
That's how I do it anyway, but I'm notoriously lazy when it comes to those important finishing off bits like flats and darks, so someone is sure to have a more effective method.