You write a BSIP and submit worker proposals via the Bitshares Github page. Best place to start is bitsharetalk.org forum and pm xeroc or any of the other lead devs. The pay is good and the work is interesting, great people!

Here's some more details, you get paid $125/hour (USD) to fix bugs via a bounty system:

https://bitsharestalk.org/index.php/topic,24755.0/all.html

https://steemit.com/cn/@lzmlam/ui-a-record-about-bitshares-ui-wp-biligual

https://github.com/bitshares/bitshares-ui

I think something like 11 bugs/bounties have already been awarded!