Tutorial: 01 Basic Frankenmeshing

It depends on the project. In some cases you can decimate the LOD 0 to reduce the poly count a bit and use that for your LOD 1. In other cases doing this can cause cut number mismatches and/or problems with the joint list that lead to mesh distortion in the game. Often, if the changes you've made to the mesh are not really drastic, you can get away with not making drastic changes (or any change at all) to the lowest LODs. Before making the decision on what to do about your lowest LODs put your item into the game and zoom away from it. Zoom out all the way and see if you can detect any major difference in its appearance when you're far away from it. If you can't detect any major difference you don't really need to change the lowest LODs...they only show up when you're zoomed out.

LOD 1, LOD 2, and LOD 3 are the lower level of detail meshes that are seen when you zoom out. The most important of these is LOD 1 because it is seen close up enough in game to see details pretty well. LOD 2 and, especially LOD 3, are seen from far away.

In the case of your project I think if you cloned the vest item and used this as your base that you might be satisfied not changing LOD 3 at all. In the game that LOD is seen at such a far distance that all fine detail is lost. This would cause the grey sleeve texture to be applied all down the arm of the Sim and it will look odd in Studio. However, in the game you cannot see the Sim wearing that LOD up close. You may even be able to use the original LOD 2 without feeling dissatisfied with the in-game appearance. That would leave you with only LOD 1 to deal with. If the LODs are similar enough structurally you can decimate the LOD 0 lightly and use that for your LOD 1. If they aren't you may have to make LOD 1 using the same method that you used for LOD 0.

Well, I had to restart the project one more time, to put effort on joining both meshes (on my first try, there were seams difficult to correct), doing a less complicated UvMap, understanding the texture, bumpmap and RLES files, and doing the LOD files using the decimating option.

orangemitten, I decided to go with the decimating route, decimating each mesh starting with Lod0, decimating it to get Lod1, repeating it to get Lod2 and again for Lod3. So far so good, I'll test the outfit ingame today, and If all goes well, I'll have my very first creator studio here

Thanks goes to untraditionalnerd for this tutorial, orangemittens for her advice and multiple tutorials (I found your decimating tutorial here, among others) and everyone here in Sims 4 Studio, without your help, this outfilt would exist only in my head.

EDIT: tested in-game, and the LOD1,2 and 3 show seams (fortunately LOD0 looks so good as the pic). I'm not happy with the result, so... maybe I'll start making each LOD mesh as I did the 0 one (a total pain...but I won't stop being so close to the goal). I thought about leaving the mesh of the old Maxis vest, but looks weird with the texture. But I'm not gonna give up this project.

I'm referring to the strange white patches I mark in the picture. Notice how Lod0 is good, but as I zoom out, the white patches are noticeable, specially with the gray shirt.

I was just preparing the .package in onedrive for posting on creator studios 2 hours ago... well, that was just before I noticed those errors. here ya go. You can test it ingame if you like.

EDIT: Removed the link, the mesh was fixed one day after I posted this.

Holy mackerel, I did realized that I skipped completely the UvMap1 step, the Uvmap1 on my lod0 mesh is a mess. Should I try to fix it? and since I'm asking about uvmap1, what texture does uvmap1 use? I'm sure that is not the texture one, that's for sure.

EDIT: nevermind, I found the uv_1 tutorial on this section, using the body txture, I see that some map sectors are falling into white parts of the body texture, maybe that's causing the strange white spots on my mesh while zooming out? I'll modify it and see.

I'm glad to hear you got it going...it came out looking great!