I've downloaded a skin for the sims 3 that I really like, however it only comes as a non-default. I'd like to make it a default skin, however I can't find any tutorials on this specifically (most tutorials are very vague when it gets to the skininator part of making skins). I don't want to change anything about the skin (so I shouldn't have to do anything in Photoshop), I just want to switch it from non-default to default. Is it possible for this to be explained step-by-step, semi in-depth? I don't have any experience when it comes to cc creating.
Thank you (in advance)!

Default skins are basically... what I explained here in this tutorial when creating default replacement textures: http://modthesims.info/t/579498
(Really should fix the mesh bit there :P... getting to it!). So now, you know what a default replacement does and how it's created. Now... to get it all from non to a default replacement. I'd say, try pulling the skin dds files from the main game files. Not that this is super necessary, since someone already did this for you: http://modthesims.info/t/351328 (See, Extracted basic skintones). All these dds files? just import them into a clean S3PE file, yep all of them.

So basically:

1. Download those extracted skins
2. Open your downloaded package with the skins. Extract ALL images, put them in a folder where you know which is what.
3. Open a new S3PE window, all empty, nothing...
4. Go to resources> import> from file...
5. Import all REFERENCE/EXTRACTED skin (So the ones you downloaded just there) dds into the file.
6. Now, right+click on one IMG file (Usually I start at the top), click 'replace' and replace this with the RIGHT Img/dds file that you extracted from the non-default package. so afbody goes with afbody (see file names), etc.
7. proceed to do this with every other IMG resource in the package you made.
8. Save the package and test!

That's literally it. I guess when you're stuck you can always refer back to my texture tutorial or maybe the vague ones (which I can understand if you're super new to this, they are really vague, but when you get the hang of it, you'll get one of those 'ohhh I get it! :D' moments.) So skininator is, in my opinion, only needed if you wanna make non-default replacements.

Also! If you encounter files that the non-default doesn't have, like say, a super black yet a few white highlighted texture, or anything that, just keep it as it is. Don't delete it, don't replace it. Just replace the textures you have. If you have windows 10, it should be easier to see which file you're replacing with what since windows 10 does have a DDS preview support, which Windows 7 (and I think 8) doesn't.

Thank you, this is very helpful! I was playing around yesterday trying to figure it all out, so I was opening the skin in S3PE and skininator to look at the files, and I noticed something that was weird (or that I thought was weird, anyway). The skin, in game works for everyone of all ages. However, when I looked at the files, it only has textures (or whatever they're called) for adult female face and body. So when I was trying to figure out how to change the skin to a default myself, I basically opened the skin in skininator, clicked 'default' and saved it like that. When I put it back in my game, it worked as a default... but only for adult females. The skin no longer worked (it just didn't exist) for any other age or for males. Is this going to be an issue if I try to make it default again? (the proper way this time).

That's not your fault, nor the game! This is because the creator simply didn't bother to make anything for the 'lower ages'. So this either means you'll have to photoshop this into the skins that the creator didn't touch yourself or accept it as it is