How Should I Proceed with Installation

OK,  I have the following installed : 
-Base game (from DVD)
All others are from Games4theworld:
-High End Loft Stuff
-Outdoor Living Stuff
-Master Suite Stuff
-Seasons
-University
-Island Paradise

I did try to install Late Night but it said to uninstall everything, etc etc.  But I am thinking I might as well get the rest of the EPs and SPs from these guys (because they're awesome).  I assume I need to uninstall everything and install in order?  Would love some smart people insight!  Thanks :-)

I noticed that Late Night had weird instructions that involved uninstalling everything... Is that still the case?

OK... I'll try again.  I installed it 2 weeks ago and it messed up all of my games!  I'll go by that link you shared

Thanks so much!  I appreciate you responding.  I wasn't sure how truly active the forums were here.  Actually quick question - can you use your downloads with legal copies - like will the game still run OK if there's a mix?

I actually own almost all of the EPs and SPs... I just didn't install any of them onto this particular machine.  I wanted to test out Island Paradise and then kept adding to it.  I'll just stick to yours on this one then   Appreciate your responses, again!

When I try to install Pets, it tells me to insert disk 1.  How do I get this to stop?

//EDIT

Steps 2 and 3 have the same file name... And I did step 2 - applied all cracks per directions... and half of my games are missing now.

Helper Edit: No double-posting.

-cat2006house

OK the thread : https://games4theworld.forumotion.com/t1171-must-read-how-to-install-the-sims-3-correctly

Steps 2 and 3 download the same files to my computer... Same exact everything...

*sigh*  Feel free to boot me out of here.  Wow.  I waited an hour for that to download... twice.  Yeah, I fee like an idiot now!  I hope it fixes this error : 

//EDIT

OK, sorry about the double posting... I didn't start a new topic about the Pets issue... I only commented on someone else's...

Helper Edit: No double-posting. Use the "Edit" function to edit your post instead for additional information.

-cat2006house

Yes I have mods... Everything was fine until I installed the new update when you guys released it.  The game runs just fine, so should I be OK?

OK, this might sound stupid, but the only things I have aside from your games plus my retail Sims 3 is downloaded versions of everything in the Sims 3 store that I got from piratebay as well, plus something else to organize the folder (from modthesims)...and I can't think of what that program/file is called.  So those are the things to remove?