Pokemon is fun, right?

The method is called Swap breeding

Just put a Ditto in the daycare and save your game. This only works in Gen 7 games so keep that in mind. So now that your ditto is in the daycare go and grab your trusty magikarp. Now at this point there's 2 things you gotta do

1. Check and make sure both your magikarp and the Pokémon you want shiny both are from either your native region or foreign.

2. Make sure you equip your Magikarp with any breeding items that you'd also be giving the Pokémon wanted to generate your preferred shiny

Please never mess with the ditto once it gets put in the daycare or you'll ruin everything. Now simply get cracking and hatch them eggs. Magikarp has the fastest hatch time in the game. Combining Roto-Hatch and a Pokémon with flame body and you'll be hatching eggs in like 5 seconds flat per egg, it's crazy. So here's another important factoid about this, you should probably save every 30 hatches or so. So you've gotten your shiny Magikarp and you're asking yourself “Now what?” Well you need to soft reset your game. Keep track of which egg this was and just put in the preferred Pokemon. Another important thing here- if you want to hatch a genderless Pokemon then you'll need to deny every other egg. This also applies to getting hidden abilities and the like.