Two methods are available – via the myafrinic portal or by updating the whois database using the email method.
Log into https://my.afrinic.net
Go to Resources -> IPv4 Resources
Click on "+" to expand on the target allocation
Click "Add Assignment"
You need to repeat the above process for all assignments that will be registered. Any changes you make from MyAFRINIC will be instantly updated in the AFRINIC Whois database.
It is possible to send one batch job in one email (containing multiple assignments) using the procedure listed here