@aimz1810 That depends on the format of the custom content. Some files come in .Sims3Pack form, in which case they're installed like store content. Put the downloads into Documents\Electronic Arts\The Sims 3\Downloads, open the launcher, go to the Downloads tab, check off the packs you want to install, and click Install.
For .package files, they need to be installed like mods. Here's a useful guide: