First time trying to make a Mesh... what did i do wrong.

Set up blender:
- make sure you have the beta version of s4s instead of the community tested one, it is the one that accepts blender 3.3x;
- the blend you shared was made for blender 2.7x versions, so if you have any of them, delete them;
- open s4s settings and change blender path to blender.exe in blender 3.3 folder;

Fix the topology:
- go back to MD, open your file, select all patterns of the dress, change their topology to quads, increase the particle distance. In the future, you can retopologize meshes using MD's manual retopology tool( not the automatic one, it is bad), but for now its fine;
- it would be best to stitch the cuts on the sides of the dress (harder to close holes properly, legs may clip into the skirt) and split the dress in more patterns;
- export as thin, weld, no internal shapes or textures;

Study the reference:
You should always start by cloning a similar maxis item and studying it. If you will make yours function just like it does, you will avoid 99% of problems. Unfortunately, no base game dress is shaped quite like yours ( same length, no sleeves, no collar), but there are pack ones, choose either:
wedding stories - yfBody_GP11DressBridesmaidTunic
moonlight chic - yfBody_SP32DressSilk
growing together - yfBody_EP13DressBabyShowerFloral
If you dont have any, heres a package for educational purposes
Export the blend and open it. Check these things ( please actually check this things, they are hard to understand without an example in front of you):
1). Number of meshgroups. You will see three of them, with two in the calf area. Their names do not matter, only the cuts. The item is split this way to prevent legs from conflicting with tall boots. To be imported into the same package and overwrite the original maxis item, yours should be split the same way into the same number of meshgroups, each of which should have corresponding cuts;
2). if you were to look at the bottom of the dress, youd see that the hole is closed and there is no body under the clothing. The clothing has the other side only where it is visible, not everywhere. All areas of the body covered by clothing should be deleted and the holes closed. It is easier to do this step if the dress has no cuts on the side;
3). uv_0. If you were to select the meshgroup with the dress and look at its uv_0, youd see 3 things that differ from uv_0 in the blend you shared:
- the areas of the body covered by clothing are deleted;
- in their place are the uvs of the dress, not in the bottom right corner. If possible, try to avoid that corner, because everybody puts everything there and you end up with cas full of items that cannot be used together;
- the uvs of the dress do not look stretched dramatically. When you assigned a texture to the background in the uv editor, you must have noticed that the uvs doubled in size vertically. They were generated in the square uv space while CAS diffuse textures are rectangular, so they stretch to fit the image. Before arranging uvs of the garment, you must bring them back to the original size;
4). uv_1. Uvs do not fit the uv space. In your blend, the garment is not unwrapped in uv_0 at all. Presumably you accidentally skipped this step;
5). Weights. Well, the only thing you should consider is that this is a dress. Skirt areas are rigged differently from nude legs or pants. So if you are making a dress, transfer weights from a similar dress, start a package from a similar dress, not from a swimsuit or a chemise;
6). Vertex paint. Dresses are painted with two colors: 3FFF00 in the skirt area, 00FF00 for the rest. The author of the tutorial gave the wrong hex code. Instead of painting the mesh manually, you can just transfer vertex paint just like weights and uv_1;

Adapt your mesh:
- open the blend with the maxis dress. One of the meshgroups contains everything you need, so you do not have to join them. You can rename it to Reference and hide the other two;
- import the dress from md in this scene;
- select the dress and close holes (quoted text only);
- rename UVmap to uv_0, create uv_1;
- select the reference, Ctrl select the dress, Weight paint, Weights - Transfer weights, By name and Nearest face interpolated. Limit total to 4;

- transfer uv_1 from the reference. Add Data transfer to your garment, choose Reference as source object, set the rest like in the pic, apply;

- transfer vertex paint the same way but with slightly different settings;

- to make a body, you can duplicate top and bottom under the rig, then join copies or you can append a lingerie set. With this item selected, switch to edit mode, select everything with A, Mesh - Merge - By distance ( witch Merge distance set to 0,0001);
- enable X-ray and delete all areas of the body covered by clothing;
- select the garment, choose uv_0, in uv editor select the uvs of the garment, assign this template to the background, press S Y 0.5 to bring the uvs back to their original size;
- in this order: select the body, Ctrl Select the dress, join with Ctrl J;
- arrange the uvs. Do not scale uv islands individually unless the difference in texture quality is acceptable ( zippers or buttons can be bigger, the front of the dress should not be bigger than the back);
- check how the mesh behaves in cas;
- if everything is ok, bake textures ( with the body in the scene, separate it from the dress but do not delete it), make lods