VERY IMPORTANT! You must have the Latest version of Firestorm Downloaded in order for Bakes on Mesh to work.
You can get the Lastest Firestorm viewer from here: https://www.firestormviewer.org/choose-your-platform/
You need the Omega rely in order to get bakes on mesh to work on Mesh parts that have not updated to Bakes on mesh yet so you do not have to wait. Your Mesh Part has to have Omega support in order for this to work.
Link to Omega Relay is here: https://marketplace.secondlife.com/p/Bake-on-Mesh-skin-applier-Omega/14391590
excerpt: "Bakes on Mesh is a new feature to allow system avatar baked textures to be shown on mesh attachments. Currently you will need a special project viewer to use it. Bakes on Mesh does not depend on simulator code, so it should work in all regions and all grids."
This applier will put the baked mesh texture from your default avatar skin/clothing onto any Omega-compatible mesh which uses the head/upper body/lower body textures. This does not require any updates from your mesh maker - it goes on your body like any normal skin applier. The baked skin will appear as just plain gray to anyone who is not using a beta viewer, so until this goes live on all viewers, do not expect anyone else to see you properly!
ALSO VERY IMPORTANT! YOU USUALLY DO NOT NEED TO WEAR ANY BODY ALPHA WHEN USING BakeOnMesh! PLEASE REMOVE ALL ALPHAS OR BOM WILL NOT WORK.
BakeOnMesh will transfer alpha maps for your clothing to the mesh body that's inheriting it. It will also automatically hide any part of your default avatar body that has a corresponding mesh body to replace it. This means that if you are wearing a mesh body and head, YOU DO NOT NEED TO WEAR ANY BODY ALPHA for your body or head! If you wear an alpha map intended to conceal your default body, it will instead hide your mesh body (or just turn it pure red)!
The good part of this, is that future clothing intended for mesh bodies can precisely conceal parts of the body using an alpha map instead of relying on alpha cuts using your mesh HUD.
The bottom line: BakeOnMesh is a work in progress for Linden Labs. This product is intended to allow everyone to get a sneak peek at how it will go using their existing mesh bodies, and it will continue to work for you once the feature is included on all viewers! Some mesh bodies will take time to get updated with BakeOnMesh features, so with this product in your inventory you'll be ahead of the game when things are live.