From the control panel, you can create a Floating IP at any time by clicking the Create button in the top right, then click Floating IP.
This will take you to the Floating IPs page within the Networking section of the main navigation. When you have no Floating IPs, this page will invite you to choose a Droplet and assign one:
To reserve and assign a Floating IP to one of your Droplets, locate it using the Search for a Droplet, then click the Assign Floating IP button.
If you assign a Floating IP to a Droplet that was created before October 20, 2015, you will be presented with a modal message including instructions for enabling Floating IPs with older Droplets.
Once you have at least one address assigned, you can reserve additional Floating IP addresses in specific datacenters without assigning them to Droplets by following the click here link in the header text.