Just a few guidelines:
The first 9 Pokémon must be Starters
Include at least 1 family from each type
Include at least 1 family from each generation
Have at least 1 Sub-Legendary, 1 Legendary, and 1 Mythical (Refer to
https://www.serebii.net/pokemon/legendary.shtml if you don't know what equals what)
You don't need to fill in all spaces if you can't
Have fun!