You would put the ip helper address command under the Vlan interface (SVI). For example say you have hosts on Vlan5 you want to get IPs from a DHCP server on Vlan1 that is has IP 192.168.1.10:
interface vlan 1
ip address 192.168.1.1 255.255.255.0
interface vlan 5
ip address 192.168.20.1 255.255.255.0
ip helper-address 192.168.1.10
-HTH