Exchange 2k in Child Domains

Can someone please tell me what the correct procedure for this is. Example domain xxx.com has child domain yyy.xxx.com. Do I run forestprep in the xxx.com domain and domainprep in yyy.xxx.com?? This doesn’t seem to be working for me. Am i supposed to run forestprep and domainprep as xxx\Administrator or yyy.xxx\Administrator. Would be great to hear from someone who has done this and can tell me how it is supposed to be done.