Stellar.org is making available the 19 billion lumens in several rounds. The second round commencing June 27th, will consist of 16 billion lumens.
On June 26, 2017, we'll take a snapshot of the blockchain at the first block mined with a timestamp on June 26 (UTC/GMT). On June 27, 2017, we'll publish a claim page, allowing bitcoin holders to verify that they control a given bitcoin address and send that address's share of lumens to a Stellar account.
To get a sense of the giveaway math for bitcoin holders, take Rachel as an example:
At time of snapshot
15,692,500 bitcoins in existence
Rachel owns 10 bitcoins
Rachel can claim
3,000,000,000 * (10/15,692,500) = 1,911.72 lumens
Rachel owns .0000649% of bitcoins in existence at the time of the snapshot.
Her 1,911.72 lumens therefore equals .0000649% of lumens given away.