After last October Stellar is distributing again up to 16 billion Lumens to Bitcoin holders or 16% of the initial Lumens in their distribution program.
For each Bitcoin you can claim almost 1,000 Lumens.
Example:
16,400,000 Bitcoins in existence - you own 5 Bitcoins
You can claim
16,000,000,000 * (5/16,400,000) = 4,878.05 Lumens
© stellar.org
On June 26th stellar.org will take a snapshot of the blockchain at the first block mined - this will record the coin balances of all bitcoin accounts at that time.
On June 27th they will provide a link to a claim page, where you will need to complete this process:
Authenticate with Facebook
Enter your Stellar account ID
Verify Bitcoin account by signing a special message with your private key using your existing bitcoin wallet
If your Bitcoins are located on one of these exchanges
Kraken
BTC38
Poloniex
Deribit
Yuanbao
NaoBTC