The method that I use is:
1. Copy an existing civilization information, and rename it, and then paste it in civilizations_informations adding the data you want to put.
2. When you create a civilization a file with the data will appear, with a meaningless name Ex:11035jfkslclbahc, You will rename this file with the TAG you want but make it the same with the civilization information.
3. Since your civilization is registered you can change the flag to the one you want with this link https://image.online-convert.com/convert-to-png , remember that you need two sizes of pixels 68x44 and 27x18 to copy it in the folder flagsH and Flags respectively.
4. Now you are going to go to languages> civilizations and you are going to edit the Bundle.properties file so that you can write at the end the TAG of the civilization and its translation. Ex: "fers = Far Eastern Republic"
5. If you want to change the flag of a civilization, you have to copy the information of the created civilization and rename it with the Government Tag. Ex: "ger_m what is the German Empire" To know the tag of the governments just go to governments.json and look for the one of your interest.
6. To change the flag you have to convert it into the two sizes and rename them both the tag of the civilization created plus the Government Tag Ex: "fers_m 68x44 and fers_m 27x18" doing the same steps in step 3
Please answer because i couldn't answer
1.how do i add information on a custom nation (civilization that we create) [was not actually important but i need to include :P]
2.how do i change government flags of a custom nation (civilization that we create)
if you could answer please show me how