Download CAW from the official Sims Site here: https://www.thesims3.com/game/tools/world. You’ll also find a walkthrough and a tutorial there which give a good overview.
You will see there are two versions: One for patch 1.67 (and earlier, non-Origin), and one for patch 1.69 (Origin) Make sure to download the one that matches the patch level of your game! Sorry Apple-users, there is no version for Mac…
You need to be registered for downloading. If for some reason you can’t set up an account, here are the direct links to the downloads:
1.67: lvlt.thesims3.com//content/global/downloads/caw/worldtool.exe
1.69: lvlt.thesims3.com/content/global/downloads/caw/ts3caw.exe
Install CAW by clicking on the .exe. After installation you will notice a new User data folder right next to your game user data folder in your documents folder. Here you will find source textures and your worlds in progress, as well as height maps.
For Steam users: You will need a bit more work to install, please find a tutorial here: https://steamcommunity.com/sharedfiles/filedetails/?id=126164888