Click on Create Network Interface at the top of the interface list.

This opens the interface creation window.

On this screen,

1. enter a description,
2. select the subnet the interface should be on (select the subnet to which your instance is to be connected),
3. allow AWS to automatically assign a private IP address or set a custom one from the subnet IP range, and
4. associate the interface with a security group (often the same as for the instance).

Click on Create when you are done. The new interface will appear in the overview list. There you can assign a name to the interface. Check that the interface is in state available.

This will list the already created Elastic IP addresses.

In the overview list, click on Allocate Elastic IP address if you need to allocate a new address. It is also possible to assign an existing address to an interface. However, each address can only be used for one instance.

This will open the address allocation window.

In the address allocation window, select the Amazon pool (or your own pool of public addresses), and click on Allocate.

The new address will be shown in the list.

Select the address. Then select Actions > Associate Elastic IP address. A window to enter the required options opens.

In the window,

1. select to associate the IP address with a network interface,
2. select your network interface from the drop-down menu,
3. connect the public address to the private address of the interface, and 
4. click on Associate to complete the step.