Step 1: Converting a .pem key into a .ppk key
When installing PuTTY, the key generator PuTTYgen will also be installed. Open PuTTYgen → Conversions menu → Import key
Load your .pem key file and click Save private key. Click Yes in the warning. Save it wherever suits you.
Step 2: Connecting to the Machine
After launching an instance and associating a floating IP to it, convert the IP to a port as seen here.
Now open PuTTY and fill the login info:
In Session category, host name is 18.104.22.168 and port is calculated based on the floating IP.
In Connection→Data, fill Auto-login username with the name of the distribution of the machine: ubuntu, fedora, centos or debian.
In Connection→SSH→Auth click Browse... and load the .ppk file created in Step 1
Click Open to connect to the machine. If it's the first time you are connecting to the machine using PuTTy, you should see a security alert. Click Yes and the terminal window will open.