Tig's Editing Suggestions
First, if you are new, do start with the beginner tutorial and just do it as written. I know that the project might be of little interest, but meshing is complex, and even with pictures and detailed instructions sometimes it takes a bit to get things working right.
Unimesh now has 4 detailed tutorials, each covers some different important concepts, and build on the others. If you did not do all the tutorials before starting your project and find yourself repeatedly getting stuck - consider taking a break and going back and doing the tutorials as written. The projects were not designed to be 'interesting', but were chosen to present the concepts and procedures you'll need.
Whichever tutorial you choose to work with, these are some steps I use to edit meshes that I recommend.
When you choose a mesh to edit, of you will probably look for a mesh that's as close in shape to your 'new' mesh as possible. Before you get started, also check the uvmap layout of the mesh. Many of the Maxis meshes are mapped in a standard layout (matching the layout of the skintone/nude mapping). But some, like the university toga, are not mapped in the standard way at all. Others have their feet/shoes in a different location, which might work fine for you if you intend to use the same style shoes, but will not work for you if you wanted to allow for barefeet or sandals, and might be challenging for other shoe styles. The quick way to see the uvmap layout is to just extract some bmps of the textures from bodyshop by making projects. For feet, look for any outfit that has sandals, then you will see the correct location and size for 'bare' feet. Below I don't show sandals, but I show two girl's dresses that are very similar, but one has the shoes uvmapped to an area between the legs.
Note that if you are using Unimesh and go all the way through the tutorial set, you will be able to replace entire legs to put new shoes on your sim, so shoes might not be the best example of this issue. If you have uni, look at the toga to see highly unusual uvmap layouts.
When you first begin to edit a new mesh, make a single edit (a very obvious one, like pulling one point in the stomach way out) and then follow the rest of the steps and make sure your mesh imports correctly and test it in the game. Sometimes a certain tool will not work with a specific mesh, or there is a step that's not clear. It's important to make sure your mesh works before investing a lot of time editing it.
Name every one of your edits with progressive numbers – save the first as say, body1, the next body2, and so on. This will allow you to test along the way and go back to your previous working version if an edit does not work as expected.
Break your edit sessions into small edit sessions, save and import your changes and test them in bodyshop and in the game (not every edit need go into the game, but if in doubt, also test in the game). These extra steps can save you a lot of heartache, because sometimes meshes with odd behaviors cannot be easily repaired.
-- If you are using UNIMESH -- for development/testing purposes - first edit your mesh with a single morph and get that working perfectly. THEN start again with full morphs, and read in your finished gmdc, copy the comments and rename it to be the main mesh. Then duplicate it and modify it to make the fat morph (and again if doing pregnant). At any rate, working on the single morph version will make it much easier for you to do all the testing and fixing (more about that follows) and then make the changes for fat/pregnant.
Once you are testing in game, really give your mesh a workout. First do some simple walking and sitting on chairs and couches (they are slightly different). Make sure those poses all look GOOD. If you're not sure how it could/should look, dress a sim in a similar maxis mesh and examine its behavior, and see if your mesh displays in a similar way.
But that's only the beginning - Then test it by having your sim jump up and down on the couch. Pause the game when they're in various poses, and look how your mesh behaves. Spin all the way around the sim and make sure things are looking right. Then have your sim dance and do the same, pausing in different poses to make sure things look good. If you do notice an odd display issue, if it's minor (like a foot through a long skirt hem) dress a sim in a similar maxis mesh and examine its behavior - you might see that same flaw, or you might see that it can be created w/o the flaw. If you see things like harsh wrinkles, odd (uneven) distortion of your uvmap in certain poses, of other odd bumps that appear - recheck at your bone assignments in that area, comparing with a working Maxis mesh, and also double check that your uvmapping is nice and even.
If you run into some odd behavior of your mesh that you cannot resolve, consider posting asking for help (with screenshots, and as many details as possible).
This is a complex learning process. There are a couple different programs, parts to extract and rebuild, and all kinds of spots where things might not go as expected. Be patient and keep trying, and you can get it all figured out.
If you do run into severe problems, the first thing to do is to start over and try again - even experienced meshers sometimes need to do that. The second thing is that when you ask for help, include all the details you can - let us know exactly what plugin and programs you are using, let us know what steps you took, post a couple screenshots to show what the problem is.