you may need to find the joists top and bottom to fix the head and sole plates to.
if you make the frame on the floor it has to be smaller than the height of the room or you will have no chance of etting it upright, fix a second sole plate to the floor and lift the frame onto it.
if you cut all the uprights very tight you may end up lifting the celing/putting a bow in the floor and the previously tight upright will become looser each time 😉
if you do not have a 6′ level handy you can use a plumb line to fit the door framing and the lining, measure an even gap from the line to avoid the need to shoot the door to a bowing lining.
if you cut a spacer to the appropriate length all your studs centres will be great and it stops them drifting whil you fix them in place
a big g clamp is great for twisting bent studs as well