It was pretty easy. While the instructions are in Japanese, the diagrams are very clear and easy enough to follow. The trickiest bit is the shutter assembly, which you need to check out as soon as you put it together before you add more of the bodywork.
There are a few translations and guides out there on the internet which might help out with that too.
Just check everything over step by step and you should have no problems.
It's a fun camera to use, obviously not a sophisticated optical instrument but the images are pretty neat all things considered.
Here's two from my last batch.
gakken011 by
Antony J Shepherd, on Flickr
gakken021 by
Antony J Shepherd, on Flickr
Both taken with Ektar 100.