This is a 4-wheel tender for 00 gauge, for use with the matching locomotive body (https://www.shapeways.com/product/6GZDQGDSF
) which must be purchased separately.
To complete the tender you will need two pairs of 12mm wheels (Hornby part number R8098, for example) and the "NEM pocket" coupling of your choice (Hornby part number R8268 will match the one at the front of the locomotve).
The tender is hollow, and it is a good idea to add weight to the inside. An easy approach is to pour in plaster. Plaster of Paris is rather weak, but there are plenty of stronger plasters available from model and craft shops.
Fit the wheels by placing one pointed axle end into the hollow behind the axle box. Then ease the frames apart gently (by pulling on the axle boxes) to get the the other end in. Do not apply pressure to the thin part of the frame. The wheels will be a little stiff to begin with, but after a little use they will start to turn freely.
The model can be spray painted, or brushed with several coats of thinned hobby acrylics. Gloss acrylics are more durable than matt acrylics. After painting the glossy finish can be flattened with a coat or two of semi-gloss varnish. Use black for any visible elements (the NEM pocket, the drawbar, the insides of the frames) underneath the tender.
This post on the RMweb forum has more details about painting and lining: