Create CNAME for Sub Domain if A Record exists?

Hello All,, How i create Cname if this subdomain have A record exisiting? example : Domain = example.com A Record = xxx IN A 1.2.3.4 Cname = xxx.example.com IN CNAME custom.example.net (another domain/external domain) I try to create but i have trouble "CNAME and other data" please help me,,, Thank you