Hi Jim,
I'm in the middle of doing the same job. I'm not a carpenter but I'm doing it myself after getting structural advice from my brother, who is an architect.
All you need to do is to make sure the boards you are placing above the joists are of exact equal thickness... 35 mm in my case. Six inches is ok to raise them once you fasten them well. I chose to raise mine 4 inches (100 mm) and I managed to comfortably fit in double sheeting of insulation roll without squashing it.
From my experience so far there are 7 points I would make.
Firstly, you don't need to raise the joists all the way across. Only raise them where you will walk. So effectively, you are going to raise the timbers in the middle (between the rafter-supports); about 2 meters wide; or wider maybe, depending on your house. You wont ever be walking out to the extreme corners so you don't need to put in flooring there.
Secondly, measure your joists meticulously. If you get the measurements right, then you've made an important start.
Thirdly, remember to mark any spaces to cut out to allow any wires or water piping that might need to run through the joists (currently running over your initial joists).
Fourthly, fasten the joist extensions onto the original joist below by using standard brackets. Two or 3 max each side will do.
Fifth, and this is an important one. After putting in your first two joist extensions, put in what's called a strutt to connect them. By this I mean a plank of timber linking the two joists. Cut it to measure the distance between the inside of the two joists. A piece of timber four inch by one inch will be fine. Position the strutt on the inside of the two joists flush with the top of the new joists or even a centimeter or two below. Screw in a long screw throught each joist and into the end of each strutt to hold it in place. Strutts serves a special purpose... they stops your new joists from swaying. It will make them rock-solid. Stagger the placement of your strutts for easy access.
Sixth. (and maybe I should have said this at the start) When you're doing your work up in the attic, have a wide board of timber up there to kneel on that will allow you to get relatively close to your work. And it also gives you somewhere to leave your screws, brackets, screwdriver-drill etc. Dont try and do the work by balancing on joists. That's messy and your knees will be in bits. Once you've three or more new joists in you can bring up some flooring and you'll begin to work a lot quicker (and you'll get a great sense of achievement seeing a few boards up). Don't screw them in yet though, you'll need to move these around as they'll be your movable floor, as you progress from one end of the attic to the other.
Seventh. When you're putting in the actual insulation rolls, just roll them under the strutts. That's a very quick process. Then you're ready to place in all your floorboards.
Let me know if that helps. I'm based in Dublin, Ireland.