Mods, I don't think I can do this.

http://modthesims.info/wiki.php?title=Game_Help%3AInstalling_Sims_3_Package_Files%2FSetup_and_Files
Here's a tutorial.Click the blue download button and download the mods folder, then copy and paste it into your The Sims 3 folder.
And you should be all set.To make sure it works, launch the game.The game should "skip" the opening scene, as the folder has a "no intro" mod pre-installed.

You will need a program like winrar to extract them.
https://www.rarlab.com/download.htm
click either 32 or 62 bit.

YOU SHOULDN'T copy sims 3 mods into sims 4, and Vice Versa.I believe mods work differently for each game and if you have a sims 4 mod installed into sims 3, it could cause a compatibility issue I think.

There is no danger of "messing up" your installation. Mods live in and work out of the TS3 user game folder in Documents, not in the Program Files. The base game and EP/SP installs are only concerned with the Program Files, they have nothing to do with user added content like CC or mods. The worst thing that can happen is that your user game folder becomes toast (this is not typical, most though not all mods are easily removable) and will need to be replaced, which is probably why whatever instruction set you were using was telling you to make a spare copy of that folder for safekeeping.

You do not need WinRar or 7zip to decompress regular .zip files like the Framework. You do need one or the other for .rar and .7z files, so it depends on how the files are offered by their creator. I prefer 7zip because it's free and open sourced and it works on all three types of compressions. WinRar will nag the user to purchase a license after the trial period is over even though the program continues to work.

For me I had to have winrar or 7zip to extract it, because my computer does weird things with files, like it literally opens it.The box pops up "Open with" and wants me to select a program.So I just assumed winrar was required.Probably just my funky computer.

That's the only bad part about winrar.I just hated 7zip because my computer constantly wanted to use it for everything including my photos and screenshots(even items that didn't need to be unzipped, again my computer is weird so...XD)

Thank you! I'm all set to get mods!

I'm so happy! Overwatch is working perfectly. It's deleting vehicles and turning off radios. I'm really enjoying this game again! I've noticed there are a lot more sims showing up to lots now. Is it due to the mod? They aren't stuck somewhere anymore?
I'm thinking about getting Error Trap. Although I don't think I've ever had a game I couldn't load, except when I had disabled some ep's on accident and it told me I didn't have some of the things from said ep's. I've just read a lot of simmers saying they couldn't play without Overwatch and Error trap.

@RosettasStoned - ErrorTrap captures ("traps") errors that might otherwise go unnoticed or cause the game to crash and reports on them by way of script logs. It also performs a fair amount of cleanup of junk (trash) data along the way, thus helping to keep our ongoing game saves leaner and less polluted. Garbage collection, meaning data not actual objects our sims need to throw away, is an important part of programming but EA left this element out of TS3 and, probably but I wouldn't know for sure, other Sims games. It certainly has an impact on the version we discuss here and this is why we say that our long-running games could likely never survive as long as they do without it.

There is a mindset out there, I've even seen players say this at NRaas, that ErrorTrap should only be added after an ongoing game experiences issues in an attempt to clean them up or generate the script logs to bring to us for interpretation. And that is a bit like never brushing one's teeth or flossing and then doing both five times in a row before a dental checkup. That really won't help much and your dentist will be able to see what you are (not) doing almost right away. ;)