On the menu, on different part's most common on spawned options, you have a CUSTOM X option, can be Custom ped, Custom vehicle. This are no more that a file with models names that can be used later.
To start adding models first you need the next page:
FOR PEDS (MALE, FEMALE, ANIMALS, HORSE ETC): https://www.rdr2mods.com/wiki/peds/
FOR VEHICLES: https://www.rdr2mods.com/wiki/peds/
FOR ANIMATIONS: https://www.rdr2mods.com/wiki/pages/list-of-rdr2-scenarios/
Now as example if i want to add a Ped to a custom list, to quick access later on the menu i go to:
1.- FikitRDR2\Custom (The FikitRDR2folder should be on you AppData\Roaming ( Menu Crash )).
2.- I Select on this case CustomPeds.txt cause i want to add a Ped (Human).
3.- I Search the model on the website, and get the NAME MODEL (on this example i gonna select: **CS_crackpotRobot** this model is for the robot.
4.- I put the model on the file.
5.- I save the file.
6.- I Reinject the cheat to load the changes.
When adding a model name into any of the files you need to doe on the last line, and add a (comma) ' , ' at the end. As example before i add my model the file looks like:
A_M_M_ARMCHOLERACORPSE_01, A_M_M_ASBBOATCREW_01, A_M_M_ASBMINER_02,
WHEN I ADD MY MODEL THE FILE LOOKS LIKE:
A_M_M_ARMCHOLERACORPSE_01, A_M_M_ASBBOATCREW_01, A_M_M_ASBMINER_02, CS_crackpotRobot,