Getting the plans

Firstly this is extremely late into the game, when you reach Chapter 7/8 you need to get shares to a certain % and you need to aquire a boss item for each girl. To raise and lower shares just spam battles on the colosseum or do quests but be warned when you raise one share others will fall.

The dungeon plans are unlocked by completing information events in each city during chapter 7.

All of these require you to get the Tough Enemies recipe for the remake system which is crafted from the normal boss monsters from the same dungeons.

Nepgear


Firstly to get Nepgear you need to get Planeptune's shares to 50%, you only need 30% to get the plan but you cannot craft her till your at 50%. Then go and get the God Soul Item from the NeoGeo Front Dungeon from the boss mob Tiamat, then wait till chapter 8 and you will be able to craft her

Uni



For Uni you need Lastation at 50% and then go and get the Magical Reactor 4 item from the Factory 459 Dungeon from the boss mob Deus Ex Machina, then wait till chapter 8 and you will be able to craft her

Ram & Rom



Ram and Rom you get together thankfully so they are quite easy, you need Lowee at 50% and then go and get the Small Whale Bell item from the M-Frontier Dungeon from the boss mob Saint Whale., then wait till chapter 8 and you will be able to craft them

Final Words and Advice

Once you have the plans you may craft them, be warned you can only craft 2 of them on a single playthrough or you will lack items for all of the game, so i would advice crafting Ram and Rom and then either Uni or Nepgear depending on your preference, they do carry over to NG+ though so 2 runs will get you them all.