You need the DNA Splicers, and have both Kyurem and Reshiram/ Zekrom on your team.
>In Pokémon Omega Ruby and Alpha Sapphire, the DNA Splicers are obtained in the Gnarled Den, which is accessible when the player has both Reshiram and Zekrom in their party while soaring above Hoenn.
Bulbapedia on the location of the DNA Splicers.
Note that Kyurem can be caught only if you have both Reshiram and Zekrom. After you obtain the DAN Splicers Key object, you can simply use it to fuse your Kyurem.
Here's a video, just in case, for the DNA Splicer Location.