The bottom row sits below the home keys and slightly to the right. The finger goes down, the hand does not.
a → z s → x d → c f → v and b
This is the most awkward of the three rows: the finger has to curl. This is exactly where you are tempted to look down, or to grab a key with the wrong finger. Once a wrong finger becomes a habit, that key has to be relearned later, and that takes longer than getting it right from the start.
Z and X are rare letters, but C, V, B, N and M are everyday ones, so the row itself is anything but rare.
If you keep getting one letter wrong here, pick a pace at which that letter comes out right as well, and hold it for the whole exercise.