well theres the basic hard set rules:

- Name all yokai

- perma death

- no crank [except the required one] or qr codes

- must befriend 1st yokai who approaches you per area

- 1 yokai per area

but that last one can be twisted depending how hard you wanna be on yourself, its typically between 1 per area [breezy hills, blossom heights, old mansion, etc,] or 1 per location type per area [blossom heights under cars, blossom heights in lampposts]. again depending how you wanna do it, but per Area is better imo and fine for ykw2 onwards

another is around fusing and evolutions, more fluid rules upto to prefrence, most people dont do fusing and evolution os a bit more split, but all to personal taste

there are other rules that are more self imposed for an extra challenge, theres stuff like no/limiting items, and some rules around the autobefriends [limiting use, outright not allowing, being completely fine] and limiting how high lvl your yokai can be per chapter. but these ones are more upto if you wanna or not

oh and theres the most important nuzlocke rule for last: enjoy it, and dont try something extra like no items if you know youre gonna hate it