You just need to measure all the areas - check out where the seams are. Draw out pattern pieces, add a little for ease and add a seam allowance. If it has a loose cover already you could unpick that and use as a pattern. Try making one with some cheap fabric first before you splash out on 10 metres of anything fancy!
You could also try googling 'sofa recover tutorial' or something similar and see what that turns up I know I've seen them around various crafty blogs. Ruths idea sounds good (not sure if I'd quite be up for the challenge though )