Time Sieve needs to be on the battlefield first, and when you begin your upkeep with Thopter Assembly as the only thopter on the battlefield, it will create five 1/1 tokens, which you will use as sacrifices to your Time Sieve
In all cases, the card or effect will say something along the lines of You may cast this spell for [cost] rather than pay its mana cost, or You may cast this card from [zone] for [cost] instead of paying its mana cost. In other words: An alternative cost is something you can pay rather than paying the spells mana cost
It's always a creature card in other zones, regardless of your devotion to its color
If I didnt include one of your favorite cards from this set, please let me know in the comments section below, or over Draftsim Discord
Since flying is one of the best ways to dodge blockers, these commanders often make your evasive creatures more dangerous, more consistent, or more rewarding to play