Yes you could start off with flex-pipe; it should be limited to 3'. OR you could start with 2 or 3 adjustable ells; they will make any angle for you from straight, to 90 degrees. I really prefer the adjustable ells.
To attach the duct to the furnace plenum (the sheet metal box, usually on top of the furnace) you cut a precise 6"round hole and insert an "A-collar" then pipe or fittings can be attached to the a-collar. Turn upward with an ell and terminate with a 6" 4 x 10 universal boot in the attic floor. Lay a 4 X 10 floor register in the boot and you're done.
Caution: Working with sheet metal presents a lot of sharp edges, when you get cut on it the zinc (galvanizing) gets into the skin and causes a burning sensation, wear lether gloves.