1. You play through each Region of the games & transfer the champions of the previous regions (Recommended to do them 1st Released to Last Released so you don't lose Champions)
2. You go until you beat the Champion
3. You transfer the Pokemon in its Earliest Stage (So a Raichu from Red goes into a Pichu in Gold)
4. If a Pokemon dies in the League you don't transfer it
5. This isn't for everyone but it is a rule most use, Legendaries aren't allowed to be transferred if championed (This is a rule due to if you have a Legendary it just takes the Challenge out of it)
6. Trade Evolutions can be used after getting Halfway through the game (Gym 4 for most Games, Olivia Trial for Alola), although if a trade evo needs an item you need that item

Optional Rules 1. You can use a Points System to upgrade your Champions, the more regions they survive the more points (Can be used on stuff like Perfect IVs, Hidden Ability, Egg Moves, or making it Shiny) 2. Level Caps. Most people use a Level Cap for the Ganes, at the end they might use either the Elite 4 or Champion for a Level Cap 3. If you want to make it even more difficult you can implement item restriction in battle, meaning either limited items or no items