https://deetron-sims.tumblr.com/frankenmesh

All these programs are free except Photoshop.
Joining Meshes in Blender 11-16
Editing Alpha/Texture 17-23
Bump Map 29-34
RLES/Specular 35-38
Editing Shadows 39-40

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Open Sims 4 Studio, under 'CAS' click 'Create a 3D Mesh'. Find your top and hit shift to select all the swatches. We want a swatch as close to white or grey as we can find for easy editing.
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Go to the 'Meshes' tab and export 'LOD 0' only and save it somewhere you can find it. I usually make a new folder at this point. (If you don't have the correct version of blender, sims 4 studio will prompt you to download it at this point.)

- Using the same process as Step 1-2, except this time we will be exporting a dress. Once you get used to joining meshes you can use parts of dresses, or pants, or even 3 or more meshes at a time. At this point, we are just going to add a new top to this dress.

- Open your 'base' mesh - the dress we just exported. Make sure you are in 'OBJECT MODE' and append the top we exported in Step 1. Turns out our top has 7 cut layers, select all of them!!! Your top may have more/less.

- Ctrl + Click on all the top's cut layers. Make sure you don't select the dress' layer. You can toggle them on/off with the little eyeballs to make sure you select the right ones. TIP: To make this easier on yourself you may join layers on the .blend before you append.