Firstly, if the goal is to just get rid of the ugly maxis outfits, I suggest using hiders on those, and then just modifying some custom clothes by setting the creator string to 00000000-0000-0000-0000-000000000000. That's a lot easier, and you don't have to hunt down and stompinate every item using the old mesh. I only replace meshes for NPCs and other uniforms that you can't change in less intrusive ways.
OK, very quickly, this is how I do it:
Find the CRES resource for the mesh you want to replace; the original maxis one, from the original game files.
Use the SimPE Resource Finder, search for sensible keywords (if you hover over the clothing in debug mode in cas, you'll get a popup with some indication as to what to search for). In this example I'll replace the male maid outfit with Simsbaby's tuxedo mesh.
Search and double click on the CRES to open that package. Right-click on the resource and extract it, just put it somewhere safe for now.
Then, if needed, find the textures that go with the mesh in question, extract those too (extract the whole TXTR resource, not just export the image).
Create a new file in SimPE and import the resources. Save.
Open the mesh you want to replace
with in SimPE. View the Resource Node (cres) in plugin view, go to the reference tab, and copy down those four numbers.
Now, replace the original numbers in the maxis CRES with the numbers from the custom CRES.
Don't do it the wrong way around: you should make no changes to the custom mesh at all, just to the exported resources.
Commit and save.
Replace the textures with something that will actually fit the new mesh.
Done.
I hope that made sense; it's almost my bedtime, and I've had a few...
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