First you have to get the plans, they are given to you by Chirper at the beginning of Chapter 5.

The plan require 512MB and 3x Crimson stones, there are no guides online (that I could find) that explains where the crimson stones are.

All locations of Crimson Stone provided by Tamhar92:

1. Neo-Geofront (Harvest Change on and off) (Story dungeon)
2. Neo-Geofront Depths (Harvest Change off) (Story dungeon)
3. Naasne Volcano (Harvest Change off) (Unlockable dungeon)
4. Naasne Volcano Depths (Havest Change off) (Unlockable dungeon)
5. Noma Cave (Harvest Change on) (DLC dungeon)
6. Nights' Dimension (Harvest Change off) (DLC dungeon)


If you dont have Naasne Volcano, Noma cave or Nights dimension in chapter 5 you then have to get to Chapter 7 and go to the Neo-Geo front dungeon (story related, you cant miss it). The Stones will be here:


In this image I have picked the stones up, they should be 1m infront of Nep in the image.

Now craft that plan and enjoy the Colosseum!


Tamhar92's sources: wikiwiki.jp/neptune-re1/ (japanesse website) and http://www.gamefaqs.com/vita/708361-hyperdimension-neptunia-rebirth1/faqs/70360.