Some custom stacks come with installers and some with a few instructions. If that’s the case, then run the installer or follow the instructions.
Otherwise, installing a stack is pretty simple: you need to put it into a special folder, then Stacks will see it the next time you launch RapidWeaver.
The folder is:
Note: ~ represents your user folder.
Before you place the stack into this directory, make sure it’s unzipped. Usually Safari does this for you, but if you’re using another browser you might have to double-click on the zip file to unzip it. When its unzipped you should have a folder with a name that ends in “.stack” — when you do, you’re all set.
After you place the stack into the Stacks folder, Quit and then relaunch RapidWeaver. If all went well the new stack should appear at the bottom of the list in the Stacks library.