I wanted to access a RHEL machine from Cacti, so I needed to install and configure net-snmp on there first.

yum install net-snmp

After installing net-snmp the snmpd.conf needs to be configured:

vi /etc/snmp/snmpd.conf

Adding the following enables access from other machines on my local network:

view   systemonly  included   .1.3.6.1.2.1.1
view   systemonly  included   .1.3.6.1.2.1.25.1

rocommunity public 12.34.56.78/32

rocommunity  public default -V systemonly
rocommunity6 public default -V systemonly

Next I setup the snmpd service to start on boot:

systemctl enable snmpd.service
systemctl restart snmpd.service