It is in the guides:
Step one:- Making your Campaign folder.

Make a new folder in \data\world\maps\campaign\ called my_campaign and copy the entire contents of the Imperial campaign into it. Also copy the contents of \data\world\maps\base\ into it.

Now go to \data\text\ and copy the file imperial_campaign_regions_and_settlement_names.txt and rename it my_ campaign_regions_and_settlement_names.txt Note: This file goes in data\text folder, NOT the campaign folder

Now when you run Rome, you should see the option to play a Provincial Campaign in the Single Player menu. Your campaign should appear and it should be a playable copy of the Imperial Campaign.
but its not 100 percent clear, so thanks anyway