Hello
. Well, I just checked your files. You have a lot of problems going on in the mesh. Clothing is a really hard thing in the Sims 4 since it has too many variables that can go wrong. For this reason I have not yet done any my self, maybe someone more experienced will give you more details if you give it time.
However, what I can tell you for now is the following.
1. You need to give your mesh what's called a cut number. This number is what tells studio where to import the mesh in the program. This depends on the cloth item you used as a clone for yours. In the case of the item you cloned. It has two different groups if you open it in blender. Two different cut numbers. I would recommend you use one clone item that has one group, and 1 cut number. For example this one.
Now you need to go to your mesh, and assign the correct cut number to the mesh. That first.
2. Your mesh need a UV map where you can use a texture. In adition to this. EA decided to make things harder and used a single uv composition for all the items in Cas. This means that your cloth texture and your map, needs to be in a specific place in the uv map.
3. You need to make a second UV map, this one used for the sliders in cas.
HERE4. You need to vertex pain the mesh to work properly.
5. You need to assign weight to the mesh. What is usually done is transfer from a similar EA item.
You can find a tutorial for that specific things under the "http://www.sims4studio.com/board/25/cas-pose-tutorials". I'd recommend you to read the step by step for begginers. It doesn't matter if is an accesory item, it will surely help you undertand the basics. It is a long process but I'm sure you will get it! As you can see there you can find many helpful tutorials. As it would be huge to make a step by step for this particular item you share. If you have problems with the tutorials you can surely ask too!.
Mainly, the point is that your item needs a lot of work and is imposible to share it as a brief answer.
If you need more help searching for tutorials let me know but there's a great amount of them you can read in that section just linked above.
Good luck!