Hi there, I know this is a very probably well asked question, but I wanted to convert some shoes from sims 3 to sims 4, but I have no idea how to do that, nor do I how I know how to extract the meshes from the sims package. Camkitty allowed me to try to convert her wolf feet to sims 4. I have no idea where this would go, because It deals with extracting a sims 3 mesh from a package on this site and then converting it somehow to sims 4.
Help please! None of the guides I have checked have really answered or helped me.
I'd also like to try to convert some tails to the sims 4 too, but I suppose it the same process for anything conversion.

Use S3PE to extract the item's meshes and textures: http://www.simlogical.com/s3pe.htm . You'll want to extract the GEOM files (meshes) and the _IMG files (textures).

If you're a Milkshape user, you can use S4 CAS Tools to convert the TS3 GEOMs to TS4 GEOMs. Clone a pair of shoes with CAS Tools and then, in the Package Import/Export tab, open the package you just saved. Use the 'Export S4' button to extract each mesh (all the LODs) to use as 'Reference Meshes' with each of your S3 mesh LODs. Then using the GEOM Tools tab/GEOM Conversion, put each of your TS4 reference meshes (one by one, one for each LOD) in the 'Original S4 GEOM' slot and each of your TS3 meshes in the 'S3 GEOM' slot, to convert them. Going back to the Package Import/Export Tab, use the 'Import' button to import each of your converted LODs to your new package. Test in-game to see if they line up with a TS4 sim's feet - if not, reshape the original TS3 meshes in Milkshape, export as TS3 GEOM and do the conversion process again. Finally, make some recolours - Sims 4 Studio is a good tool to use for that (link below).

If you're a Blender user (or if you don't have a meshing program or have never used a meshing program), you can use Sims 4 Studio to do the project, both for meshing and recolouring. Blender is freeware and Sims 4 Studio works together with it, enabling you to work on any necessary reshaping to your mesh in Blender using TS4 bones (no back-and-forth conversions needed). There are loads of good tutorials on the Sims 4 Studio website and this very simple one on creating accessories for TS4 is a good place to start, in order to get the basic idea of how to create TS4 CAS stuff. I'm not aware of a shoe-specific tutorial, and I've never made shoes so I can't advise on specifics myself, but you can ask questions over on that site and they are usually quick to answer and always helpful.

Moving to CAS Parts since you're specifically talking about shoes and tails which would be CAS (Create-A-Sim) parts.

When I try to do the conversion, it says error, "Index was out of array" or something. I extracted the meshes from the file, (really just export them, there is no extract option.) What am I doing wrong?

Index out of range is what I sometimes get when my group names don’t match. This is a shot in the dark since you don’t mention at what point this is happening, but with a conversion this seems a likely cause.

If you mean you're getting an error message when you try to convert your TS3 mesh to TS4 with CAS Tools using the 'GEOM Conversion' tab, this does sometimes happen (it happens to me often) and I'm not sure if it's a bug in the tool or if it's something we're doing wrong. If that is what happened to you, try the 'Auto Conversion' tab instead, which has always worked for me. The one issue with that tab is that it doesn't handle body morphs, but as you're doing shoes and/or accessories, you should be OK. Input your TS4 reference mesh and your TS3 mesh like you did with the 'GEOM Conversion' process (leave all the boxes at their default settings) and click Go, and see if that works.

Alright, so nothing is working, the meshes don't show up in milkshape at all. I'm really at my wits end with this, is there anyone who is open to converting them to Ts4 for me? I think the only kind of CC I could do is for Ts2, anything other than that seems to just elude me, and i feel if I ask questions on super simple things, i'll get laughed at and belittled. Argh.

Aww, you won't get laughed at or belittled here. As you now say it's Milkshape you're using, did you download and install the Milkshape GEOM plugins? If you don't have those, you won't be able to get your .simgeom into Milkshape, so it won't show up. If the plugins are installed, then under the Import menu you'll see an option for Q-Mesh Sims 3 GEOM Importer. If you do have the right plugins, then a clearer explanation of 'the meshes don't show up in Milkshape' will help - what steps did you take to extract them in .simgeom format with s3pe, and what steps did you take to get them into Milkshape?