Can somebody help me with step-by-step list of how to create a multi signature 2-of-3 transaction?
Actually, both receiving funds at and spending funds from a multi-sig address is already supported.
You can find the code to do so, plus comments, by Gavin:
It seems some partial implementation exists. Quoting Gavin:
Generate a multisig address:
addmultisigaddress returns the multisignature address. Be a little careful, the public keys are raw hexadecimal and don't contain checksums like bitcoin addresses do. You can then send funds into that 2-of-3 transaction using the normal sendtoaddress/sendmany RPC commands, or the GUI (or anything that's been updated to recognize multisig addresses).
Spend from a multisig address:
This isn't implemented yet (RPC calls mechanism). You could gather up all addresses into one wallet, and then you would be able to spend the funds.