There’s a few ways you can go about her corset.
Perhaps the easiest would be to simply purchase an off-the-rack underbust corset and modify it as needed. Even though you won’t likely be looking to actually reduce your waist, I would still look for one with steel boning and a curvy enough cut that it will conform to your underbust and hips while still defining your waist and not creating gaps. Remember that the lacing should be parallel in the back! Here’s a good guide to the basics.
Another option would be to make an underbust corset. It doesn’t have to be extremely complicated, since you likely aren’t going to want to tightlace with it, but still be sure to use a firm fabric and high-quality boning – I typically recommend spiral steel for the bendy areas like the waist and flat spring steel for the areas like the center front – since a lot of cheap plastic boning can warp and twist and become very uncomfortable. Making your own would allow you to customize the fit and the detailing in a way that purchasing would not. Even a basic pattern found in the costuming section of a major commercial pattern manufacturer’s catalog would work well for this, since you don’t need tightlacing or historical accuracy. Thankfully, costume corset patterns are very common these days!
Lastly, you can make a faux corset out of a stretchy fabric and enough boning for structure, but not the same amount or type you would use for a full corset. I would recommend using a stretch interfacing on your fabric if you go this route, as it helps create a bit more firmness and control. This would be the least corset-like, but would move in the same way hers seems to. It also would take less skill in getting a precise fit, though you would need to know how to work with stretch fabrics. I would recommend a boning like rigilene for this option.
Any of these options can either have the panty area attached to the corset or as a separate piece worn underneath.
For the front detailing, I would recommend creating the neck piece as a separate item and then having it snap to the front of the corset rather than sewing in on. This will allow you to dress and undress easier than if it were attached, and if you needed to remove the neck piece for any reason, you wouldn’t need to remove your whole costume. If you have a pre-made corset or if you make a corset with a busk in the front, you can use the detailing to cover the busk and hide it. The red gems can simply be sewn or glued over where the snaps are.
I hope that helps! Good luck :]