Legends of Runeterra
Legends of Runeterra is a digital card game by Riot.
Building a Deck
Find a champion you like. Build the deck around that. Imagine how you would win using that hero, that is your "winning condition."
Select 3 copies of the main champion. Select single copies of 3 different champions around the same mana cost that work with the main champion. Or if some other champion synergizes with the main champion winning condition, take 3 copies of that.
Next, build up to 21 units that you can play without a condition on mana curve. This means you would play the unit even if there were no allies, no enemies on the board, both players were full heath and both players have empty hands.
40 cards 1 mana = 3 units that you can play without a condition 2 mana = 6 units that you can play without a condition 3 mana = 6 units that you can play without a condition 4 mana = 3 units that you can play without a condition 5 mana = 3 units that you can play without a condition note that I also consider champions you can play without a condition "units" makes 21 units with a solid curve
This leaves you with 19 cards to mold the deck to your liking. Find cards that synergize with the deck, not the best cards overall. If you want more aggressive deck, add 1-3 mana cost cards. If you want more late game deck, add more 3-5 cards. I rarely add anything over 5 mana except if it's related to the deck winning condition. It's good to have some burst and fast cards to play on defense.