The following post is lifted verbatim from reddit.  As it has active links which are useful, I'm including the url for it.

[Step by step modding-Guide]: Turn Baldur's Gate and its sequel into the single best CRPG experience you'll likely ever encounter
byu/billyblaze ingaming

This struck me as such a brilliant idea that I'm actually about to do it.

Also, Amazon has all 4 BG games for $20.  This was not my post.  If you go to reddit to use the links, upvote it.  🙂

----

Before I scare you off, let me tell you that the mod process for this game isn't as tedious as it is with Fallout 3, Oblivion or Dragon age. Leomar of the BG community - Spellhold Studios - actually coded up a kind of Wizard to help you along. All you'll do is say "Yes", "No", "Continue" and put in some checkmarks. All compatibility issues will be automatically resolved. Good. Keep reading.

I'm here to help you play Baldur's Gate. I can already hear you moaning, "but I can!", but, really, no you can't. Not in the way it should be played in 2010. I'll rephrase: I'll help you play Baldur's Gate the way it's meant to be played in 2010, tweaked&modded to hell and back and with any resolution you want. Heck, you'll even play BG1 in the BG2 engine with a giant worldmap that stretches over both games. And thanks to the efforts of the dedicated BG modding community, it'll feel as seamless as if it came out of the box that way. I'll start off with a few bullet points to sell you on the prospect:

    * several hundred new areas
    * over a hundred NPCs (as in only the ones who can join your party), which of course have their own storylines, dialogue (lots of them voiced)
    * unimaginable numbers of new spells,
    * quests,
    * monsters,
    * tactical challenges,
    * class-kits,
    * romances,
    * engine modifications
    * widescreen resolutions
    * custom-GUIs
    * and just about every bug or hiccup to ever grace the game has been squashed

You can actually play both games with their expansions in one sitting, since they will have been combined into one game, one big experience once the modding process, I'll now lead you through, finished.

Alright, that was the part where I sound like a telemarketer, now to what you'll need:

    * Baldur's Gate + Tales of the Sword Coast
    * Baldur's Gate II + Throne of Bhaal
    * BiG World Setup
    * ~25GB free on the your install drive. That's not a permanent necessity, but once everything is unpacked and extracted, it can reach those heights.

And here comes what you need to do:

    * Install both of them completely, that means you choose "Custom" during the BG1 installation and check every component and you choose "Full" during the BG2 install.
    * Don't install them into their standard folders. Drop the Program Files. Don't change the root directory names, though. I got my stuff in C:BIBGII - SoA and C:BIBaldur's Gate.
    * Once done, patch both of them: BG1+TotSC patch, BG2+ToB patch
    * Start a BG1 game, press Q for quicksave. Start a BG2 game, press Q for quicksave. Start a BG2:ToB game, press Q for quicksave.
    * Extract the BiG World Setup I linked above into your BGII - SoA folder.
    * Execute BiG World Setup.vbs
    * You'll be greeted by this piece of software, full of magic and awesome, with a question to update. Do that.
    * On the next screen, you see three directories. Even if the first two already match with your installation folder, click the [...] besides them and navigate to the according directories. Why? If you installed and patched the games according to my instructions, the [...] will turn green. Green is good. Leave the third directory intact if you don't have a good reason not to.
    * Read the right-hand text and then set checkmarks as you see fit.
    * According to what it says on the right side of the screen, choose "Minimal", "Recommended", "Standard" or "Tactic". If you choose Expert, you're utterly insane and run into a 250% chance of your computer exploding. I recommend "Minimal" for purists who are merely in it to relive their fond childhood memories of the game, "Standard" for everyone who played the games before about once or twice, "Recommended" for curious newbies and "Tactic" for people who whip the games out every year and breeze through it. Proceed to the next screen.
    * If you enjoy the modding process as much as you enjoy playing games: you probably put in the checkmark under "further customization" on the previous screen - if you did, you should not put one next to "Restrict selection to the BWP-version" (It means you can't select any mods that belong in the "Tactic" category should you have chosen "Standard", etc.). If you're generally timid about this kind of thing (nothing wrong with that!) and want to be over with this modding voodoo as soon as possible, do the opposite. Click continue.

Okay, get yourself a cup of coffee and take a breath of fresh air before we go on. I already feel bad for consuming so much of your time. But it'll be worth it, I promise! Done? Great.

The ones who chose to further customize their modding selection will now do so, and I'll just assume that you'll be okay. Once you're done with that, you'll end up where our modding newbies are already - and that's what I'll go into now.

    * You get to click a few buttons now - I hope you're as excited as I am. You see six of them, but you can ignore the bottom 4.
    * That leaves us with #1 and #2. Click the second one, let it do its thing, once done, "Continue" so you get back to the screen with the buttons and click the first one.

What happens now?

    * Mods get downloaded
    * Downloads are extracted

Once done, the real fun begins. If you click continue and "yes" often enough, it'll tell you that once you exit this program, the BiG World Install.bat will open automagically. It'll prompt you with a short series of "Yes"/"No" questions that you can answer to your liking. They all come pretty much in the beginning of the process, and then you can just watch the lines go by and feel like a hax0r badass. This will take a while, depending on your choices during the setup anything from 1h30 to 6h is possible, but you can let it run in the background and do other shit.

After this process is finished, there's not much left to do. Launch the game via "TobExLoader.exe" in your BGII - SoA directory and enjoy the game.
I can't begin to thank all the people who turned BG+BG2 into a so much better game than it originally was. Not because I don't know how, it's because there's ten sites with countless creative individuals involved. So I'll just thank the community as a whole.
I'm also not involved in the development of any mods or programs, merely a lurker who thought he'd bring the good word to reddit. If you feel like thanking the BiG World people, head over to the Spellhold Studios

Apologies for horrible formatting - not my strong suit. English hiccups occur because it's not my mother tongue. With that out of the way, feel free to ask any questions about the process in general or throw specific problems at me, that's cool, too. Enjoy, Reddit.

EDIT: If something doesn't work and you can't figure out what exactly it is, upload the logs somewhere and let me have a look at it. For the BiG World choosing/downloading/extracting process, you'll find a Logs folder within your BiG World Setup/ folder with various log files in it. If your problem occurs during the command line installation process, upload the WeiDU.log in your BGII - SoA folder. Even if you're unsure, come here and post your problem - the process is patchy and some confusion is expected. n