Find a build that tickles your fancy, and go to the corresponding download page. 

Download the .zip or .7z files. 

Once the download is complete, unzip the folder with either the built-in tool on your computer or a third-party software like 7z or WinRAR. 
(On Mac, double click the zipped folder, and Archive Utility will open. You can then copy & paste the files)​​

Copy and paste the files to your Tray folder, located at Documents/Electronic Arts/The Sims 4/Tray. Only paste TRAYITEM, BLUEPRINT and BPI files in your Tray folder!

Some of my CC builds will include an option to download a merged folder with both Tray files and PACKAGE CC files. After you unzip the folder, copy & paste the subfolder named TRAY ITEMS into your Tray folder, and the PACKAGE items into your Mods folder, located at Documents/Electronic Arts/The Sims 4/Mods. 

Load up your game, make sure Mods and Custom Content are enabled in the Options menu. 

Bring up the cheats window by pressing Ctrl + Shift + C on both Windows and Mac (no Cmd key here), type testingcheats.true, and hit enter. You should see "cheats enabled". 

Type bb.moveobjects on to enable the Move Objects cheat, which is very important for items in builds to show up correctly.

Go to the gallery, click on "My Library" at the top, select "Lots" from the filter panel on the left, and check the "Include Custom Content" box. You can further narrow it down by ticking the "created by Others" box to filter out your own builds.

Place the lot, and enjoy!

Check out the FAQ here.