I figured that a machine printing onto fabric is bound to be fairly accurate; I could rely on that to ensure that, at the very least, the right angles I draw will come out as true right angles on the fabric. Through ringing around I learned that while this turned out to be correct, there could be shrinkage of the material either lengthways, sideways, or both, due to the printing process.
There was a local digital fabric printing company who said their fabrics were treated beforehand to largely eliminate this shrinkage: Vapor Print. Just in case, I drew an extra line a couple of millimetres around each outline to enable me to use that line if the fabric had shrunk in that direction.
To have fabric digitally printed, you need to send the printing company one large image to be printed. The width of this image needed to be the width of the fabric of course, but it could be any length. The length of the image for my couch’s cushion covers was around 10 metres:
By the way, if you are wondering why the shade of green differs between the above two pictures, it's because I changed the shade after getting a trial print back from Vapor Print.
When the printed fabric arrived I measured the distances and was pleased to find that the only shrinkage present was a couple of millimetres per metre in one direction only. This was very workable.
My solution was to use tape to attach each piece together along each seam. I would then either remove the tape or leave it in there while I finished off the internal seam edges. This method worked out very well indeed - sewing seams which have already been taped up is a breeze.
Having cut out the shapes using printed lines, the edges were quite accurately straight, so just maintaining the position of the edge of the fabric relative to the needle was all I needed (with my amateur skills) to ensure the sewing lines were straight.
The only other skill I needed for this upholstery was the ability to sew zips. I found a great YouTube video that showed a good way to do it: