For painting, there are 3 main paint jobs you could do. There's the slightly lighter blue seen from seasons 14-15, which starts out pristine and gets more and more beaten up as it goes along. Then there's the repaint in season 16, where the whole prop is painted a slightly darker blue. The fresnel lamp housing remains white the entire time, up until it's lost or removed in S17. The last few stories feature the prop with a police car style lamp on top, and a black base and supports on the bottom of the walls.

After printing, you should paint the insides of the walls black first. Once that's done, insert the windows, and probably superglue them around the edges, otherwise they might fall out when you hammer it together. Paint the windows now, because it'll be much harder to do once they're inside the box. Use citadel contrast paints in one thin coat on the front and back of the windows.

Once the windows are done, you can assemble it all. Slide the walls in and try and align the doors to the holes in the base, and then hammer the base in. It's a super tight fit, and it will probably take a lot of frustration to get it on perfectly, but I've tested it twice now and it's worked out in the end both times.

Now glue in the "alignment cube" file into the top part of the poles. Put a bit of glue in the square hole for the roof, and then put it into place.

Now turn it back over and attach the little handle to the underside of the base. This will probably also take a hit with a hammer to achieve.

Put in the lamp (they're easily swapable, though keep in mind that the only time the non-fresnel lamp was used with the prop not having a black base was Destiny of the Daleks, if you're going for 100% accuracy) and then you're just about done.

Print out the signs and attach them with something like a gluestick.