Hey,
There are quite a few questions related to this on the curse website, so I wrote a small tutorial on doing this. It is already preformatted in mediawiki format and ready to go on your wiki. I am not so familiar with contributing on github so I just upload the file here...lostcities_wiki.txt

It looks like this (converted to markdown...):

Introduction

This small guide explains how to get a custom profile for the world generation. The configuration files already are nicely commented by the author of the mod, so first there will be a introduction on the structure of the config files, followed by a short explanation of how to make a custom profile.

Configuration files

Location

You can find the configuration files in your modded minecraft installation path, e.g. for FTB-DW20 it is located in '...\Curse\Minecraft\Instances\FTB Presents Direwolf20 1.12\config\lostcities'. There you find a 'general.cfg' which contains the general settings of the mod and a list of all profiles that are available in the menu of the world generation in minecraft. Additionally, there are all preconfigured profiles in the folder which have the prefix 'profile_'. So in general if you want to combine several profiles you have to open them and look at what is different from all the other profiles and then copy paste the information you want into your new custom profile.

To open a *.cfg file you can use any text editor of your choice (e.g. Notepad).

Structure of the config files

Each file starts with a commented header on what this file contains. A comment is started with a '#'-symbol and contains information on what the following line does. For example here the header for the 'general.cfg' file:

# Configuration file

##########################################################################################################
# general
#--------------------------------------------------------------------------------------------------------#
# General settings
##########################################################################################################

Each configuration file contains several 'chapters' that contain different settings. Each setting is explained using a comment above it. E.g. the setting that modifies the density of cities is:

# The chance this chunk will be the center of a city [range: 0.0 ~ 1.0, default: 0.02]
S:cityChance=0.02

For each setting the possible range and default values are given. In this case the probability that a generated chunk is a city center is 2%. To make city generation more rare this value is modified in 'profile_rare.cfg' to be 0.002, giving a probability of 0.2%.

The leading char ('S:' in the example above) in front of each setting defines the type of variable used for the setting.

    S: A string variable (used for floating point numbers e.g. 4.2)
    I: An integer variable (only accepts round numbers e.g. 42)
    B: A boolean variable (only accepts 'true' and 'false')

Chapters in the profiles

Each profile contains several chapters that contain the respective settings for the city generation (e.g. city size), explosion properties (e.g. explosion chance), city style (e.g. changes for each building type), and minecraft structures (e.g. caves and dungeons). The chapters start with a commented header (surrounded by #) and are followed by a block definition (surrounded by curly brackets):

    ##########################################################################################################
    # lostcity_xxxx
    #--------------------------------------------------------------------------------------------------------#
    # Settings related to the Lost City for the xxxx profile
    ##########################################################################################################

    lostcity_xxxx {
        ...settings...
    }

Creating a custom profile

Always keep a backup copy of your 'profile_default.cfg' somewhere in case something goes wrong.

To Do List

• Duplicate the default profile and rename it to your desired name keeping the 'profile_'-prefix. Only use one word for the filename after the prefix.
• Open the 'general.cfg' and add your file to the list of supported profiles. You can find the setting around line 43 and add your custom name at the end of the list:

    # List of all supported profiles (used for world creation). Warning! Make sure there is always a 'default' profile! [default: [default], [nodamage], [rarecities], [onlycities], [tallbuildings], [safe], [ancient], [wasteland], [chisel], [atlantis], [realistic]]
    S:profiles <
        default
        nodamage
        rarecities
        onlycities
        tallbuildings
        safe
        ancient
        wasteland
        chisel
        atlantis
        realistic
        custom
     >

This adds the option to the selection dialog in your minecraft.

• Now open your 'profile_custom.cfg'
• It is now best practice to change the occurrences of '_default' to contain your profile name instead of default, e.g. 'lostcity_default' to 'lostcity_custom'. Tip: Use the search and replace function of your text editor to make this step easier
• Check if each block also contains the profile name (in front of the curly brackets)
• Now you can change each setting to your liking. If you want to know what the settings for a specific profile are, just look the up from the other profiles. Or read the comments in the file.

Trivia

• To add chisel support, change 'S:worldStyle=' to chisel.
• To change the description of your profile change 'S:description='
• To add the realistic world generation copy the chunk generator json from the 'profile_realistic.cfg' into 'S:generatorOptions='

All good ...

'cept the last point on the Trivia section.

Namely nothing changes when you drop that JSON in that option.

Thanks :)