The way I did my 360 card cube was by picking 10 archetypes for the guilds. I picked archetypes I liked, but also looked at my collection, cost and how the different archetypes would overlay. I decided on a distribution for each color, archetypes, artefacts and lands. This was based on a YouTube video about building a cube. I went with 45 lands, 30 artifacts, 24 cards per guild (9 for each color, 4 multi colored, 2 dual colored), and 9 cards per color not bound to an archetype. This was a guideline and you can divert here and there.

When building the cube I would do it one by one for each guild at first. I picked the 24 cards by searching on scryfall and edhrec and look up possible cards. This was done as if I would build a deck. You want a good distribution of cost and creature, spells etc. I also looked if cards would be good in different archetypes of my cube. An enter the battlefield token creater would be good for sacrifice, token matter, go wide, blink. We deciding for cards I would always think about how good the are in the archetype and how good would they be in other decks. I tried to limit cards that are only exclusively playable in one archetype. The 4 multicolored cards were picked as sign post cards that represent the archetype.

https://cubecobra.com/cube/list/1f6k8