Generally speaking, you'll likely get some sort of instructions, though the specifics will vary - sometimes things pretty much have hand-drawn "IKEA style" instructions, sometimes it's wall-based or floor-based paper outline templates - basically you tape the template to the wall, put screws in the locations marked, then take the template off and put the objects on the right screws.
You normally get example photos from previous times it was shown, as reference. There's often written instructions which might have very specific measurements of distances and angles, which can be difficult to follow in 3D space. Sometimes the artist is present, and they'll say "left a bit, up a bit, turn it a quarter turn clockwise" etc.
In most cases, the artist/owner/gallery wants the work to look as good as possible, so they try and make the instructions as good as possible.
Sometimes even less complicated works have basic instructions on assembly, and best practice examples of how to move, handle, rotate, install the work etc.
If instructions are particularly lacking, a previous technician may have written up better instructions - and certainly I've done this a handful of times - especially with new works that haven't been shown before. Sometimes we make notes whilst we install, and with some photographs, that becomes the "official way it's done".
In some cases it's actually left up to the curator (and sometimes the individual technician) and the instructions are just stuff like "put the sand on the floor, then put the plinths on the sand and put the sculptures on the plinths".

We offered the options of:
a) build a support frame with a single horizontal centre baton, painted the same colour as the window frame, then mount the works to that
b) build a frame top and bottom of the window, and suspend the works on either metal wire or transparent plastic "fishing line", depending on weight
c) do actually drill holes through the glass (no guarantee it won't shatter) then put bolts through, then hire a massive scaffold and go up the outside to fix the bolts in.
d) as above, but take the window out and replace with perspex
e) install the works on the blank gallery wall over there, in the normal manner
We explained the extra materials and time needed, and they went with installing it on the wall in the normal way :)