When you’re installing custom content, where it goes depends on which type of file you have.
With details for your garden furniture (and in fact everything except custom sims), there are only 2 possibilities:
If you have a sims3pack file, it needs to go in DocumentsElectronic ArtsThe Sims 3Downloads.
It will show up in the Downloads gardening section of the game launcher for you to select and install from there. (or if you only have one file, you can just double click it to bring up the installer – more than one file and this method takes forever!)
If you have a .package file (or a .package that was compressed into an archive file like .rar or .7z) it needs to go into some new folders. If beautiful gardens you’ve already set up the required framework, just place the .package file (after extracting from the archive if you need to) into DocumentsElectronic ArtsThe Sims 3ModsPackages.
If you haven’t yet outdoor ideas set up the required framework to install package files, follow the simple guide here: http://simswiki.info/wiki.php?title=Game… – that will tell you everything you need to know. For more info on extracting packages from archive files and the required programs, you should have a look at the ‘CC Basics’ guide linked in the tutorial.
Hope this helps! 🙂