Q: So what's the deal with the new London Mulligan? When exactly do I put cards on the bottom of my library? Most importantly, how does it work with Serum Powder?
A: Starting Core Set 2020, there is indeed a new mulligan procedure for all of Magic. The new procedure is colloquially known as the London Mulligan because it was tested out at Mythic Championship II in London this past April.
The new mulligan rules work as follows:
After drawing their first hand of (normally 7) cards, a player can choose to either keep that hand or to mulligan it. If the player chooses to mulligan the hand, they shuffle that hand back into their library and draw another full hand of the same number of cards as before. The player then puts a number of cards from their hand on the bottom of their library equal to the number of times they've taken a mulligan so far. Further mulligans proceed in the same manner.
A common shortcut in this process will be to just shuffle your hand back in without putting cards on the bottom if you know you're taking a mulligan further anyway, but if you do this be sure to keep track of how many times you've taken a mulligan so far.
As for Serum Powder, its effect can be applied when you normally choose to take a mulligan, which in this case is after you've put back cards on the bottom of your deck. So if you have taken a mulligan twice to a hand with Serum Powder in it and wanted to use it, you'd first put two other cards on the bottom of your library before exiling Serum Powder and the other four cards in your hand and drawing another five cards to replace them.