It was quite layered and uneven in the back, so I simply cut off the longest layers.
Then I should have taken progress photos, but I didn't. Basically I put the wig on my own head and added a foam roll to the back. I combed the front hair over the foam roll and took twisted a piece of the hair from the back and rolled it around the foam roll. I did this while wearing the wig. When I was happy with the front I took it of and put it on my wig block and worked the back. For the back I divided the hair into four loops, and held them in place with elastic. I saved some strands of hair so that I could twist them and hide the elastics under them.
Here is the finished wig.
And here is me wearing the wig, but without trying to hide the wigline.
Now I need to construct a wig box so that I can transport the wig when I leave for Stockholm tomorrow.