This is not a real guide, but just a heads up.
When you create your new community in Nightmare, unless you finished a previous campaign in nightmare already, you will be on a fresh map without the option to choose any legacy (power/water from Builder, Favors from Trader, Ammo and weapons from Warlord or daily loot from Sheriff).
The only things you can carry from the old campaigns are the equipped weapons and everything that is in the backpack and pockets of each chosen characters. (you can choose up to 3 characters).
I recommend to choose characters with some bonuses like:
1 med per day
1 material per day
1 food per day
1 Additional outpost
Another good stuff to carry from the old playthrough would be an antenna upgrade for the Command Center. it would help a lot, preferably one antenna that gives you 2 more outposts, once you will need a lot of outposts to help in your community with the goods. - you need one character with Computer knowledge in your community to to upgrade it.
PS: Do not forget to put a bag of materials on each of the members you plan to start the campaign before finishing the previous campaign.