I have a simple character. I would like rig the legs so that I can easily control them. How can I IK rig my characters leg?
First you will need a target bone for the Ik - for example a copy of the foot bone. Make sure this bone is not parented to anything (except when you have a root/master bone - then parent it to root/master)
Now in pose mode add an IK constraint to the shin bone and setup it like in the picture. You can do this fast by selecting the target_bone first (copy of foot_bone), then the shin_bone and Ctrl+Shift+C then I. Set the IK chain length to 2.
Last step is to add Copy Rotation constraint to foot_bone.
Select target_bone (copy of foot_bone), select foot_bone,Ctrl+Shift+C then R
Now your leg is controlled with the target bone. You can also add a Pole bone for the IK constraint so the direction of the knee is defined:
You can eyeball the pole_angle, how to set it precisely is explained here: How to Set/Calculate pole angle of IK constraint so the chain does not move?