I did a video on the 12" one if you're interesed
At the end it shows how to assemble it (if it doesn't come assembled). The first time is a bit fiddly - after that it's quick and easy. The main thing is to make sure the front and back pieces are pushed firmly together so there is no gap either side of the plastic joining strip on the running track.
I wouldn't use the sandy track - although as Cypher says 15 mins max a week is probably ok - but with the hassle of swapping tracks once a week it may get forgotten. Sandy tracks are very bad for the pads under their feet and hammies can get a condition called bumblefoot which is very painful and hard to cure.
Another option people use to keep nails short is to have a stone or terracotta plant pot base under the water bottle so every time they go for a drink it helps roughen their nails. You can also have large-ish stones as a kind of small feature/rockery in a corner of the cage.
https://www.youtube.com/watch?v=VFussTX5PuE