I've been trying to know for ages and would really appreciate it if someone showed me or lead me to a tutorial.

Hi Wan-Wan,

That depends on what you exactly want. You want something close-fitting the skin like a leotard, or rather a flowing transparent dress?

I'll start with a VERY basic project (close-fitting) since it's much easier.
Since I'm working with TSRW, I am more comfortable than giving you instructions for CTU.
I would start it, pick something to clone e.g. a swimsuit or top (rename it!) and head for the textures tab. find the clothing "Multiplier", chose the edit button and EXPORT it - that will give you a 4-channel DDS texture with Alpha channel.

I hope you read the basic tutorials so you already have NVIDIA DDS tools installed in your Photoshop or GIMP?

Open the DDS in your graphics app and look for the "channels" window/tab. In Photoshop, you'll see virtually five channels, but the topmost one is combined RGB, below that there will be the actual R/G/B/Alpha channels. Select the Alpha Channel (only) - you will have a black and white outline of the clothing Pure white means opaque, pure black means invisible. Use a pure grey scale (R=G=B) for degrees of transparency.

For laces, find yourself a picture (google, scan picture...), rescale it to your taste. Covert to black&white (greyscale). Edit a copy so that the fabric is white while the space between the threads is black. Apply to the Alpha map. I suggest that you first add it to a separate RGB layer, trim it to the Alpha "outline" and then actually copy it to the Alpha. Afterwards, discard the layer and merge the greyscale you created a few step ago with the RGB layer to get a more realistic texture. (It's no all black and white, you know).

Save as DDS again (DXT5/interpolated alpha), import as TSRW Multiplier and check your creation. Fine-tune as desired and/or necessary, repeating the steps above.

Be aware that while the TEXTURES will become transparent, the MESH will not - these instructions will work rather well for a swimsuit or tight top, but will miserably fail if you try to edit a long skirt or loose-fitting pants. The mesh will stay the same, but the transparent parts will adopt the colors of the virtual underlying (fallback) skin.

If you want "true" transparent clothing, you will need a multi-layer mesh (clothing mesh with underlying body mesh) - with all of its complications. Success so far is limited on these due to limitations of all available utilities. Bloom might elaborate on that though.

Meyomey gave a very good description of making tight clothing with a transparent effect. Here's a tutorial on making 'true', multilayer see-through clothing - but I agree with Meyomey that you might want to start with the tight stuff since it's much easier. http://www.modthesims.info/showthread.php?t=447671