Hi! I used this tutorial to learn how to make my first showerproof skins: https://modthesims.info/showthread.php?t=89397
It explains how to link a skintone to nude meshes. However, you don't need to do all of that if a skintone for the bodyshape you want has already been created. Then it's much simpler:
First, get the file of the non-showerproof skin that you like, open it in simPE and export all the textures from it from plugin view. You should hopefully be able to tell which sex, age and state (fat, normal, fit) each texture belongs to (just keep in mind that the children and toddler textures are unisex and don't have fat and fit states, and some custom skins don't have different textures for each state for other ages either). Put all those textures aside for now.
Let's say you want to make a showerproof skin for Momma Lisa. Put any of my showerproof skins for Momma Lisa in your downloads folder. Open Bodyshop > Create Parts > Start New Project > Create Genetics > Skin Tone > select the Momma Lisa skin that you put in your downloads folder > Export Selected Textures > give it a name > Import to Game > close Bodyshop > get the newly created file from your SavedSims folder and put it somewhere so you can work on it. That's your new Momma Lisa skin, except for now it has the textures of the skin you cloned it from. You need to replace those textures with the textures that you extracted earlier. Open the new showerproof skin in simPE, open the textures and replace them one by one with the corresponding textures that you extracted. To do so, right-click each texture > Build DXT... > open image > select the appropriate texture that you extracted earlier > change Format to DXT5 > Build. Save when you're done.
If you want to change the thumbnail of your new skin so you can identify it more easily in CAS, you can do that too. Open the new skin in simPE > in the resource tree, click jpg/tga/png Image > in the resource list, right-click jpg/tga/png Image > click Replace > select a picture of your choice (the format has to be .bmp) > click yes & save.
And that's it. I have no idea how familiar with simPE and Bodyshop you are, if you aren't at all then this may not be of any help. If you'd prefer, feel free to let me know which skins you want for which body shapes and I can do them for you, I'll be able to do them quickly and I really don't mind. In fact I've been wanting to showerproof more skins but I'm not sure which would be most useful to the community, so getting requests for them would be helpful.