The thin flash in the centre of the chassis casting and the six rectangular holes will need to be cleared along with the triangular areas at each end of the ballast hopper sides and in the centre of the hoop at the left hand end.
I would recommend doing a dry run first with the sides and ends against the chassis. You may find depending on the thickness of the end pieces - which is a normal consequence of the open back resin casting process - that the frames of the side pieces are a tiny bit too long.
You can either extend each end of the chassis frame with some thin off cuts of styrene or trim each end of the side pieces very slightly so they are flush at each end.
Take care to ensure it is located in an absolutely central position.
Some filing and shaping may be required to get the top lip matching seamlessly at the corners where the sides and ends meet.
It can be made up as a sub-assembly which can made up on its own and them glued in place or you can built it up underneath the wagon.
Now fit the internal ballast dividers. The 3 triangular dividing segments may need to be filed to fit on the outside edges to match the angle at which the chute sides were fitted in the previous step.
If desired drill out marker holes and fit handrails with 0.5mm wire before gluing in place. The struts fit inside the moulded brackets on the hopper end and in line with the hoop at the end of the wagon.
You may have to chop a tiny bit off the end of the struts (at the right of the picture below) to make it fit. (This will be affected by how accurately the sides were centred when they were glued to the chassis at the start of the build)
The wheel should be given a slight indent to make it concave shape to be completely prototypical before gluing or soldering it onto the wire.
Do not glue the wire into the resin parts until you have got it into place and are satisfied with the positioning.
Offer the wire up to the model placing one end against a gearbox. Grip with wire in the jaws of fine pliers where the first cog is to be placed. Still gripping it with the pliers thread on the cog and tack in place with a drop of superglue which can be applied using the point of a wooden cocktail stick.
Move the pliers along the wire repeating the process with cogs and brackets taking care to ensure both the brackets are orientated with the flat upper surface in the same position - these are the primary contact points for gluing the shaft assembly on the wagon body.
When you have fitted the final cog in the just off-centre position then you can trim / file the wire to the correct length - there should be a very slight gap between the two halves.
Small triangular areas of flash will also need to be cleared from the bogie side castings.