It's quite simple.
1) You need to get the Poké Transporter and Pokémon Bank apps on your 3DS.
2) Put all Pokémon you want to transport from Black in Box 1. Remove any Pokémon you don't want to transport.
3) Open Poké Transporter with your Black cartridge inserted and proceed to transport.
4) Close Poké Transporter and eject Black. Insert Ultra Sun.
5) Open Pokémon Bank and look in the Tranport Box. This is where your Pokémon should be.
6) Move all Pokémon from the Transport Box into an empty Box in Ultra Sun.
7) Close Pokémon Bank and launch Ultra Sun. Your Pokémon should be in the Box you put them in.
That's how you transfer. I hope this guide helps you.