1

Open a terminal window in Ubuntu, or log in if you are converting the keys on a remote Ubuntu server.
2

Type "sudo apt-get install putty-tools" at the terminal prompt to install the PuTTY tools. Enter your password when requested by sudo, to run apt-get with the root permissions needed for installation.
3

Convert the public key by typing the following at the terminal prompt:

puttygen keyfile.ppk -o id_rsa.pub -O public-openssh

Where "keyfile.ppk" is the PuTTY .ppk file to convert, and "id_rsa.pub" is the name of the converted key file.
4

Convert the private key by typing:

puttygen keyfile.ppk -o id_rsa -O private-openssh

Where "keyfile.ppk" is the key file to convert, and "id_rsa" is the private key file. PuTTY stores both the private and public keys in the .ppk file, which is why you use the same file to convert both keys. You can now use both keys with OpenSSH.