Massage the public key into openSSH format by doing the following
Remove lirst 2 line
Remove last line
Join all lines into a single line
Add 'ssh-rsa ' (remember the trailing space) at the start of the line
Copy public key to unix ~/.ssh/authorized_keys file or append to the existing file content
Use PuTTY to select the private key to ssh to the remote server
NOTE
If using Java library such as JSch to connect using private key, need to export private key into openSSH format first. Load the private key just created into PuTTYGen and select convertion -> Export Openssh Key option. Use the openssh format private key in Java code to connect to ssh.