Please visit the Cacti website at http://www.cacti.net/download_patches.php If any patch has been released, you will find installation instructions there.

As an example, please find patch installation instructions for cacti 0.8.6j here. Do not apply those patches to recent releases!

wget http://www.cacti.net/downloads/patches/0.8.6j/ping_php_version4_snmpgetnext.patch
wget http://www.cacti.net/downloads/patches/0.8.6j/tree_console_missing_hosts.patch
wget http://www.cacti.net/downloads/patches/0.8.6j/thumbnail_graphs_not_working.patch
wget http://www.cacti.net/downloads/patches/0.8.6j/graph_debug_lockup_fix.patch
wget http://www.cacti.net/downloads/patches/0.8.6j/snmpwalk_fix.patch
patch -p1 -N < ping_php_version4_snmpgetnext.patch
patch -p1 -N < tree_console_missing_hosts.patch
patch -p1 -N < thumbnail_graphs_not_working.patch
patch -p1 -N < graph_debug_lockup_fix.patch
patch -p1 -N < snmpwalk_fix.patch