How do you upload Pokemon from the Virtual Console versions of R/B/Y to the Pokemon Bank?

It isn't available in the games yet. Support for it will happen as part of an update to Pokemon Bank at some point in the future for Sun/Moon Compatibility.

Here's the excerpt from the press release.

Major updates are planned for Pokémon Bank to work with Pokémon Sun and Pokémon Moon.

Once Pokémon Bank has been updated, you'll be able to use it to transfer Pokémon you've caught in the Nintendo 3DS Virtual Console versions of Pokémon Red, Pokémon Blue, and Pokémon Yellow into your copy of Pokémon Sun or Pokémon Moon.

1. Download Pokemon Bank and Poke Transporter from the Nintendo eShop
2. Put the Pokemon you want to transfer in Box 1 of the PC in your RBY game
   • This can be a huge hassle as you can only move Pokemon to different boxes by withdrawing them, changing boxes and depositing them again
3. Save your RBY game
4. Open Poke Transporter and confirm that you want to transfer everything
   • Poke Transporter may notify you that certain Pokemon couldn't be transferred
   • Mewtwo, Zapdos, Articuno & Moltres must be at least Lvl 50
   • Ditto must be at least Lvl 15
   • Starters must be at least Lvl 5
5. Open Pokemon Bank
6. Go left to the Transport Box and move your Pokemon into your ORAS or SuMo game
7. Save and close Pokemon Bank

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Poke Transporter will generate stats for fields that didn't exist in Gen 1. A couple notables include,

• The gender will be random
• The ability will also be the hidden ability, if a Pokemon has one
• The transferred Pokemon will randomly have 3 perfect IV's and 3 random IV's.
• The Pokemon's nature is determined by the Pokemon's experience points.