Lose something every day. Accept the fluster of lost door keys, the hour badly spent. The art of losing isn't hard to master.
The poem continues in a rather pedantic tone, instructing readers to practice losing things by losing different insignificant items every day.
The speaker tells us to "accept the fluster" (2.4) that such losses bring, presumably so we eventually stop getting flustered by them at all.
We don’t know about you, but most of us are very, very familiar with this "fluster" – you know, the small-scale apocalypse of leaving your keys somewhere, then having to turn the whole house upside down (only to discover, inevitably, that you left them on the *&$^&^%$ subway or somewhere else).
We learn that more abstract things, like time ("the hour badly spent" [2.5]) can also be counted as a loss – for example, when an hour that really probably should have been devoted to that paper due two days ago is instead spent eating Pringles and watching Heroes. Um, not that we’ve ever done that.