How to install Sims 3 expansions - I'm utterly confused

I have an issue on how to install more than one Sims 3 expansion. Last time I tried to install Pets, I then deleted Showtime and then it went fine and now works, but... I would like now to install Sims 3 Supernatural and don't really know how to proceed.

I know how to install a base game (I have it) and one expansion. But more than one... it confuses me.
For now I have on the computer a base game and Sims 3 Pets Fixed version from games4theworld.
This is what I did
Base Game
- mount the .iso file with Daemon Tools
- Autorun the game
- Type the serial code from the download .txt
- install the game
- update it manually with a patch in the folder
- open the #Crack folder and put TS3.exe and TSLHost.dll in the actual game Bin folder. Then when asked, rewrite/replace.
- delete the desktop shortcut for a sims 3 Launcher and instead go to the game folder, send TS3.exe to desktop and create a shortcut.
- Click on that shortcut and start the game.

Sims 3 Pets Expansion:
- Autorun the game
- type serial code from download .txt
- if the set up asks you to update, do it
- install the game
- put the files from the #Crack file (TS3W.exe and TSLHost.dll) to the base game Bin folder and rewrite/replace
- delete the launcher starter shortcut for the game on desktop and instead use an TS3W.exe file

- start the game

- avoid the launcher at all costs because it could recognize game as unoriginal.

Okay, this now works just like it were original (so I guess I did everything perfectly right... or haven't I?). But now... What do I have to do when I want to install Supernatural expansion?
Is it just the same like I did with Pets or is there any difference? Since I have one expansion installed (and it is a fixed version), what should I do with crack files? Where to place them not to ruin something (as the game works perfectly now)? When installing the expansion, should I use Daemon tools or I just use Autorun (to install it) from the start? Then...should I update anything if asked to?

I'm afraid that if I go with the same steps that I went with Pets with another expansion, that I may ruin something or game won't be able to start. What exactly is the point with these crack files, how do you know which one to use for a shortcut (when you have more than one or two expansions) and where do you place them for each expansion pack?

Another question... What is the difference with installing first expansion and later ones? if there's anything I need to know, I'd like to know before I even think about installing, since I ruined the game when trying to add more expansions and I hate when this happens.

Thank you allot for any advice on that matter!

Thank you very very much, I did it just as you said, and from then on the game works like new! 
However I'm wondering about that Ultimate Fix... now I have found Late Night expansion (FULL) by Games4theworld. 
As I used the Ultimate Fix when I installed other expansions (have many there, but not this one as I haven't found it back then), I wonder... if I install it now, would I just have to download Ultimate Fix again and do the steps, or do I have to do something else? Thank you!

Okay, I did all of that. It works perfectly fine! Haven't really experienced any problems since with any expansion or stuff packs and thanks for an awesome download site, the downloads all work perfectly fine!