We're back... or, should we say, we've never really left - because the banana brats have circled around you now and are clamouring for their map lounder than ever. From what you can tell, they like their banana, "yay," but insist that this be a "real" game with a "real capital BFC." I mean, gimmie a break, right?
(We're picking up where we were at the end of the last tutorial... if you've closed your command prompt since then, you can get back to where we were by running this script:
run_script "tutorials/t3/t3.civ4mc"
We'll talk a little more about scripts later.)
Making a BFC in the worldbuilder is easy enough. We'll just take one of the starting sims we generated from Tutorial 1 and edit it into something like this:
We make sure to add all modern-era resources somewhere because, like, we're totally sure Team Banana will stick around that long. 
Then we bring it into Civ4MC, and use extract_starts to get rid of the dummy AI civ.
import_group "tutorials/t4/bfc.CivBeyondSwordWBSave" => $bfc
extract_starts $bfc
debug_group $bfc
(again, here is a sample debug window for all code in this tutorial)
Start0 is the layer we want. Let's copy it into $the_real_banana and raise it above the background so its tiles will over-write the backgrounds'. Looking at the debug window, coordinate 4,6 should be a good center for the BFC; however, we'll move it to 2,4 because we want its center to be at 4,6, and the move command will refer to its origin at the lower-left corner. b. Let's also crop away all that extra ocean in the background. Moving switftly now:
copy_layer_from_group $bfc.start0 => $the_real_banana.bfc
move_layer $the_real_banana.bfc 2 4
# bring the bfc to the top
increase_layer_priority $the_real_banana.bfc
# the --rename_final flag renames the final layer of the flat group
# to have the same name as the group, i.e. $the_real_banana.the_real_banana
flatten_group $the_real_banana --rename_final
crop_group $the_real_banana 0 0 13 16
Ah, now our banana is "legit" and ready to turn into a script. But, one thing first. When this is a script, our banana will re-randomize every single time the script is invoked. Instead, let's export @random_field to a table like so:
export_mask_to_file @rand_field, "tutorials/t4/random.txt"
And we can build our script like this, which I've saved at tutorials/t4/banana.civ4mc:
# generate our banana!
import_mask_from_ascii "tutorials/t3/circle.txt" => @circle1
new_mask_from_shape *circle 17 17 --centerX 8 --centerY 8 --radius 7 => @circle2
mask_difference @circle2 @circle2 --offsetX 5 -offsetY 2 => @banana_mask
# but now lets make it random!
import_mask_from_table "tutorials/t4/random.txt" => @rand_field
mask_intersect @banana_mask @rand_field => @random_banana
new_weight_table >= 0.90 => %banana,
>= 0.87 => %luxury,
>= 0.84 => %plantation,
>= 0.81 => %strategic,
>= 0.76 => %landfood,
>= 0.70 => coast
>= 0.01 => %any_land,
=> %bananaland
# now summon the demon!
new_group 25 25 => $the_real_banana
generate_layer_from_mask @random_banana %bananaland => $the_real_banana.background
# rivet on the BFC
import_group "tutorials/t4/bfc.CivBeyondSwordWBSave" => $bfc
extract_starts $bfc
copy_layer_from_group $bfc.start0 => $the_real_banana.bfc
move_layer_to_location $the_real_banana.bfc 2 4
increase_layer_priority $the_real_banana.bfc
# and we're done!
flatten_group $the_real_banana
crop_group $the_real_banana 0 0 13 16
return $the_real_banana
Only about 30 lines, including comments - not too shabby, right? Now.... to make them dance!
fliplr $the_real_banana
debug_group $the_real_banana
fliplr $the_real_banana
debug_group $the_real_banana --add_to_existing
fliplr $the_real_banana
debug_group $the_real_banana --add_to_existing
fliplr $the_real_banana
debug_group $the_real_banana --add_to_existing
...
Just kidding... you'd have to be truly B-A-N-A-N-A-S to do something like that... *tugs collar* 
*ahem*... The only real new thing here is that "return" statement at the end, which makes run_script produce a result. It's sort of like a function, if this were a programming language. A return statement is not needed in scripts, but it's pretty handy because it allows the run_script command to return a value. Thus, we can do this:
run_script "tutorials/t4/banana.civ4mc" => $player1
run_script "tutorials/t4/banana.civ4mc" => $player2
run_script "tutorials/t4/banana.civ4mc" => $player3
run_script "tutorials/t4/banana.civ4mc" => $player4
run_script "tutorials/t4/banana.civ4mc" => $player5
We ran the script 5 times, and generated 5 bananas! As an aside, another handy thing we could do is immediately funnel the result of the script with the '--debug_result' flag.
run_script "tutorials/t4/banana.civ4mc" --debug_result
Anyways, we don't need that now because we're totally home stretch; we just need to combine them into one group and move them around a bit. Let's set the mood!
run_script "tutorials/t4/banana.civ4mc" => $player1
run_script "tutorials/t4/banana.civ4mc" => $player2
run_script "tutorials/t4/banana.civ4mc" => $player3
run_script "tutorials/t4/banana.civ4mc" => $player4
run_script "tutorials/t4/banana.civ4mc" => $player5
new_group 45 18 => $banana_bunch
copy_layer_from_group $player1.bfc => $banana_bunch.banana1
copy_layer_from_group $player2.bfc => $banana_bunch.banana2
copy_layer_from_group $player3.bfc => $banana_bunch.banana3
copy_layer_from_group $player4.bfc => $banana_bunch.banana4
copy_layer_from_group $player5.bfc => $banana_bunch.banana5
# stack banana till the morning come!
move_layer_to_location $banana_bunch.banana2 9 4
move_layer_to_location $banana_bunch.banana3 18 8
move_layer_to_location $banana_bunch.banana4 27 12
move_layer_to_location $banana_bunch.banana5 36 16
# --rename_final_layer renames the final layer to the same as the group name, e.g. $banana_bunch.banana_bunch
flatten_group $banana_bunch --rename_final_layer
# note we can also set arbitrary colors with this set_player_data!
set_player_data $banana_bunch 0 --player_name "Banana Bill" --leader "Montezuma" --civ "Inca"
set_player_data $banana_bunch 1 --player_name "Banana Alex" --leader "Shaka" --civ "Arabia"
set_player_data $banana_bunch 2 --player_name "Banana Felipe" --leader "Genghis Khan" --civ "Zulu"
set_player_data $banana_bunch 3 --player_name "Banana Eric" --leader "Stalin" --civ "Mongol"
set_player_data $banana_bunch 4 --player_name "Banana Ray" --leader "Mao" --civ "Egypt"
# we can also set the speed and/or starting era with set_settings
set_settings $banana_bunch --size "small"
# wrap can be set via set_wrapX and set_wrapY, FYI
debug_group $banana_bunch
export_sims $banana_bunch
And that's it, we're done!!!! Here's our monster, the perfect toroidal banana nightmare. I put this script at tutorials/t4/bunch.civ4mc, if you want to look at it. Note that setting player info and the game settings are pretty easy.
I started by sizing the main group to 60 by 60, and spaced out the bananas 11 by 11, but then quickly realized that was way too spacey. (and way too big to post a screenshot of in this tutorial page) Because of the toroidal wrap, we could bring in everything a lot closer. Now it's the perfect size, I think - way, way too small to be acutally playable by normal people, but perfect for these goofballs, considering that they'll probably abandon the game after 40-50 turns anyways. At least this way they'll get some action!
With great reluctance, the devil lets you go... and also, it turns out that's it for this tutorial. Hopefully, you can find your way around using the built-in help commands, which, remember, are:
help search_string # for searching
command_name --help # for getting a full description of the command
list_layers $group
list_shapes optional_search_string
list_groups optional_search_string
list_weights optional_search_string
list_terrain optional_search_string
But of course, don't hesitate to ask for help if you have any trouble!