The Shape Fix – Preparing
Shapes in SL have permissions: Mod-OK or No-Mod, Copy-OK, and possibly Transfer, and these permissions define how much control you have over the shape file.
If you buy a commercial shape, it is usually No-Mod, so you cannot directly edit, copy, or export it, but there is a trick that still lets you effectively “edit” even those looks. I’ll get to the trick later.
Shapes that come bundled with heads and bodies are usually Mod-OK, Copy-OK, and sometimes Transfer-OK, making some of them full-perm.
Any shape you create yourself in the viewer is full-perm: you can copy, modify, transfer, and even sell it as part of your own avatar styling business.
Basic: Make a Shape
To create a fresh shape in SL, open your Inventory (Ctrl‑I, toolbar button, or top menu).
Go to the system folder named “Body Parts,” and, to stay organized, consider making a subfolder named “Shapes” for all your custom shapes.
Right‑click the folder where you want to store your new shape and choose “New Body Parts → Shape.”
SL will create a brand‑new shape item using all the system default slider defaults, which you can then customize.
To prepare for splitting a shape into separate head and body pieces, right‑click the shape you want to work with and select Edit, which forces your avatar to wear it and opens the Shape Editor.
From there you can tweak your look manually, but the real power comes from a lesser‑known trick that lets you add a new head or body without wrecking the part you want to keep.
A Powerful Shape Trick: Export

Wear the shape of the existing head and body shaped as you want. I suggest you do a ‘Save As…’ giving it the name you plan to use for the new combined shape.
Read more… page links below.