I just made a loose cover for my sofa, but it is a very simple shape, no curves-just right angles.
It took me two days to plan/measure as it was my first time and half a day to cut/sew it all together.
My tips would be:
When you measure don't add on too much for ease. I took my measurement, added on seam allowance and then 10 centimetres for ease on the seat area. And then another 10 centimetres. It works but is far too big and doesnt look as nice.
Make sure you have enough material. I found I didnt have quite enough and had to add a strip of another plain material down the centre of the seat and down the side.
Measure
Measure
Measure.
I drew a little 3d picture of the sofa with all the measurements and labelled each separate piece e.g. arm, back A B C D E etc. Then I worked out the sizes for each individual piece, drew them out and wrote them down separately with seam allowance etc
So A: 120x80 (124x84) - picture of A
Then I could see clearly exactly what size I needed to cut. As I cut the pieces I wrote the number on them.
Sewing the thing together was stupidly easy following the diagram, although I did sew a few pieces wrong and have to do some unpicking. I did each piece separately e.g. back, top of arm, side of arm, as it worked better for the sofa.
Dont do it in white fabric!
Choose washable fabric, I got some cheap curtains from IKEA that worked very well.
Planning was the key for me and in retrospect I really wish I had sewn it together loosely at first so I could have corrected some of the errors much more easily.
Good luck!