How to block something

[USAmma]

I have graduated from socks to a baby kimono! I have to knit the two front pieces, the back, and the arms separately and then sew them together. I can already tell with my one front piece that it's curling around and not setting flat. I am sure it needs blocking but not sure how to do it. Also, do I block each piece and then sew them together, or do I assemble it first and then block it?

[flapjack]

Given that it's a baby kimono, I would press it: that is, iron it with a steam iron on a low heat through a damp towel- to make it easier to seam. I'd then wash it, and damp-block it properly then.

[USAmma]

Thanks!