You start with 20 life.

Each of these spells must be played on your turn.

Mox Ruby is free to play with a 0 casting cost. Tap that (turn it sideways) to add 1 mountain (red mana: 🌄) to your mana pool. You don't remove it from the board.

You have 1 mana in your mana pool: 🌄

Black Lotus is free to play with a 0 casting cost. You can turn it sideways, remove it from the board into your graveyard pile, and add 3 of any color to your mana pool. Choose green for forests: 🌲🌲🌲

You now have 4 total mana: 🌄 🌲 🌲 🌲

Channel costs 🌲🌲. Cast that, remove 🌲 🌲 from the mana pool, and pay 19 of your 20 starting life, so that you gain 19 colorless mana (can be used for most spells). Place Channel into the graveyard pile.

Your mana pool will now have a total of 21 mana: 🌄 🌲 + 19 colorless

Fireball costs 🌄 + any amount of mana you decide, up to how much you have available. So you then use your remaining 🌲+ 19 to do 20 damage to your opponent.

You win with 1 life remaining.