Hi.
I'd like to start by saying that even an experienced clothing creator will have a hard time making that item functional because of the sleeves. Items with such sleeves should be made with arms down and you will have to manually edit the weights in the sleeve area. If you want to start making clothing, you should choose something very simple like a tank top. If you want to make only this item, I suggest to not even bother because, while adapting clothing is not hard, you need to be aware of a lot of things. I do not think there is a tutorial that will teach you everything you need to know, would take weeks to make one.
But anyway
About working in 2.9, you can do everything there and then, instead of exporting as obj, you simply save the file, open 2.7 ( or 2.76 ), and append the mesh from the file made in 2.9 instead of opening it.
About maps:
Diffuse - the color information ( so, yeah, the main map ), default size 1024x2048
Shadow - the shadow the garment casts on the body. You must import it only in .dds ( some bug that creates black outline along the border of the shadow). The rgb channel should be white with soft grey shadows, the alpha should be black with white only in places where the grey shadows are. Note that there should be no blurry edges in the alpha. Please clone any maxis item and check its shadow map to know exactly what I mean. Default size 1024x2048
Specular - this map comes in two files, i'll call the first one specular and the second specular.mask. The rgb of the first file defines the
type of shine the garment will have. The brighter the color, the shinier the item. The alpha channel dictates the intensity of the shine as well as the areas of it. The specular.mask, never know how to word it, defines where exactly the specular of this item will appear. For example, if the specular.mask of the dress if fully black, the moment you put on shiny tights on the sim, the shine from the tights will appear on the dress. But if the mask matches the alpha channel of the diffuse, this will not happen. The default size is 512x1024.
Normal - not sure why it is called normal in ts4 when it is technically a bump map, but this map is used to add fine details like wrinkles, yeah. This map is tied to the geometry, so if the item you cloned has a square normal map that contains only the body region, then so should yours.
Here is the tutorial that shows how you can bake it from high-poly mesh to low poly.
Here's the one ( scroll down to Bump map ) for using an image for this.
Emission - this map can make your item emit light like alien suits do. In almost all cases it is blank by default and you do not need to touch it.
To make your cc compatible with HQ mod, you must double the resolution of each map (beside normal and emission, it does not matter in this sense).
Basically, if you do not know what any of your maps should look like, just find a maxis item that resembles yours, export all maps (preferably as .dds ) and study them, both rgb and alpha channels.
A bit more about vertex paint.
The sims 4 uses deformation maps for morphing. Those deformation maps come in two versions: skin-tight (00ff00) and robe-like (3FFF00). Robe-like variation is meant for skirt-like bottoms, sometimes you can find it between the breasts. All other clothing and shoes from the neck to the toes are generally vertex painted with 00ff00.
There are also special colors for accessories (007F00 ), hair, bangs, hairline and facial hair, mermaid tails, and so on. For example a tank top will be painted with 00ff00, a dress with 00ff0 and 00ff0 and 3fff00. As always, what you'd need to do is find a similar item and sample the colors or just transfer vertex paint.
Keep in mind that there should be no sharp transition between colors that activate different types of variations of dmaps.
Also, you'd need to transfer weights and a uv_1 map.
