In its simplest form, this is a fairly straight forward task. If you view the file /etc/sysctl.conf, you will see several attributes with their appropriate values, these are the values applied at startup.
This file can be edited directly with a text editor, then the values reloaded by executing sysctl -p
These values can also be changed at runtime by modifying the values under /proc/sys. For instance, in /etc/sysctl.conf there is an attribute named net.ipv4.ip_forward, this attribute can also be viewed or modified as /proc/sys/net/ipv4/ip_forward.
To view the running value, run cat /proc/sys/net/ipv4/ip_forward
To change the running value, run echo 1 > /proc/sys/net/ipv4/ip_forward
The kernel attributes and values available to change can be found by either browsing the /proc/sys folders, or by running sysctl -a