Decompiling a map
Get the VPK
First we need to find the VPK of the map we want to decompile
You can find a guide on how to get the VPK here
Open the VPK
To open VPK files you will need Source 2 Viewer
When you have Source 2 Viewer the VPK file you can simply double click it and open with Source 2 Viewer
Or open Source 2 Viewer and click File > Open > Select the VPK
Once you have opened the VPK right click the (workshop id).VPK > Decompile & Export > select a folder you wanna export to
Now go inside the maps folder of your exported folder and open up that VPK, open maps > .vmap_c > Decompile & Export export to the maps folder where you opened the VPK
Create a Workshop Addon
Open Counter-Strike 2 Workshop Tools (if you don't have them see this guide) > Create New Addon > name it what you want
Then go to steamapps/common/Counter-Strike Global Offensive/content/csgo_addons/(the addon you created)
Move all files from your exported folder to your addon
Open your addon in Counter-Strike 2 Workshop Tools
Go top right on Tools > Hammer (Map Editor) > then top left File > Open > maps/the map file.vmap
Top left again File > Build map... > Final Compile > Build
If your materials are blank/gray, that means they did not get decompiled correctly