With the new Daemon codex, there are now options to use chariots for our daemons.
I wanted to get a chariot for my Slaanesh herald. I'm using all old (last edition) daemonettes, but I'm going to use the new daemonettes as the heralds, as they're slightly bigger, and that sets them apart.
Step one, aquire parts
I will need:
2 Mounted Daemonettes (last edition version)
1 Dark Elf Chariot box
1 old-style Daemonette driver
1 new-style Daemonette Herald
A Banner or Icon
Some random decoration parts
Step Two
In step two I use my dremel tool to remove the daemonettes from their steeds, and to remove all the Dark Elf markings from the chariot parts.
I use an engraving bit first:
I don't recommend doing this without the flex-tube attachment either:
And, then go over the surfaces with either a file, or a wire-brush bit on the dremel (for places the file cannot reach):
Here are the results:
Step Three - Green Stuff
I then used some green stuff to fill in the gaps on the steeds, and to make them little saddle/yoke pads. I also made a slaanesh symbol for the front of the chariot.
Note the indentations on the saddlebags. I made sure that the yoke would fit over the steeds in position, and made their setting deliberately. This way, I can paint the steeds separately, and the parts will fit back together then painted.
Step Four - The Chariot
Next, I built the yoke and chariot body. Although, in order to get the crew in, I didn't glue it in place yet. As I'm going to use pins to keep the crew in place, I want to be able to remove the chariot walls to drill the holes for the crew positions.
Step Five - the crew
Next up, I added the crew. Fortunately, the dark elf chariot driver's arm and armour plates look like a perfect match for an old-style daemonette, letting me use the driver arm with minimal difficulty. The herald seems like a perfect chance to use the crazy new daemonette hair, as it will make the chariot look like it's going pretty fast

I have to put the herald in at an angle so that her claws fit.
The crew are also going to be removed for painting separately.
Step Six - Details
Small details make all the difference in how a conversion looks. I added an icon (very tall, so as not to block the herald's sight), and some random chaosy bits that I had lying around, chains and tatters and spikes and stuff), as well as the wheel blades from the dark elf box.
Done
Actually, there is one more thing that I wanted to add, and that's a small length of fine chain underneath where the yokes sit on the steeds, so it looks like there's something that they're actually pulling. But, since I'm painting them separately, those have to be added later.