I started with a few bits of .5mm styrene, and some thicker plastic for the sides (I used 1.5mm). I also used a 2mm rod for the rivets and some 3.2mm tube (I actually used 4mm rods for this previously, but I've got none left).
Then I got some random wooden stuff from the bits box to use as the machine tool. And I found what I think are sails from an old windmill kit which I thought might work as vents.
And then my knife broke, causing me to stab myself in the thigh, and so you should get an adult to help you with this bit.
I cut the side panels from 1.5mm plastic, and the base and roof from .5mm. The conveyors are 17mm high, so I use that for the height of all my bases.
Then I used medium grade sandpaper to round the corners and glued the base together...
... and then put the sides and roof on.
Then I cut up random bits of plastic that fits on the various surfaces...
... rounded the corners with sandpaper, and added rivets.
Then I just glued it all together. Pretty simple.
Here's one I made earlier.
For the conveyor legs I used some sort of railway girder I picked up at the model shop, with some .5mm styrene. I used 3.2mm tubes for the rollers.
I rounded and glued the walls to the base.
While it was drying I cut up the rollers.
Then glued the legs on.