It's intended to have 20mm space inbetween each part. 
The Wave Wall Art Spacer.3mf file can be very helpful when mounting to make sure the spaces are equal. I recommend that you print 2 of the spacers so you can have on on the bottom and one on the top.

I just used double sided tape to mount it to the wall.

If you don't have a huge printer I used the “cut” function in prusa slicer and added 2 connectors. I then also used glue to make sure then didn't split apart.

I ended up using wood filler, sanding and then painting each part to get my desired finish. 

It's possible to print each part either standing up or laying down on it's side.