I hate rigging. You spend hours copying an example out of a book or off the internet only to have your rig go into spasms in front of you and the rig does not work dispite hours of work and frustration.
I tried to use Rigify in my last film only to give up because it did not seem to work. I came up with my own rig which was fine unless the character took more than 2 paces - so I used a fast cutting editting technique and the characters never walked more than two paces in any shot.
So now I am making another film and I need characters I am having an other look at Rigify. The answers of how it works are buried in a forum thread 11 pages long that starts in 2010 and is thus mostly out of date.
Here is the very simple 'how it works' which no doubt misses crucial features and breaks crucial rules but here we go:
1. Start a new project in Blender and open User Preferences under File and under Addons enable Rigify. It is near the bottom of the list under Rigging.
2. Create or import your model you are going to animate.
3. Shift A or Spacebar or however you bring up the Add dialogue and under Armature select Rigify. You will get a simple humanoid armature.
4. Your model and armature should have their feet at the origin of the Blender space (ie. where the red, green and blue grid lines meet.
5. Edit the armature in edit mode so the bones are in the right place for your model. ( details not covered here - you need to look at some basic armature tutorials)
tips i) switch on X Axis Mirroring in the tool bar of the 3D View window (press T) then everything you do to one side of the armature will be copied to the other side.
ii) a bit of bend in the arms and legs helps the IK (see note at the end)
iii) Also do not scale the armature as an object (scale the bones in Edit). Its worth doing Alt S to remove any scaling as the 'generated' armature (next step) will not have that scaling.
6. Shift A or Spacebar or however you bring up the Search dialoge and start typing Rigify. 'Rigify Generate Rig'. A new very complicated Rig will apear - it will appear in the wrong place. Delete the lold Armature. Move the new Armature over your model in Object mode. It should fit.
7. Select your model then select the Armature and do Control P. I select deform with automatic weights.
But Wait! There are no constraints! Ah but there are. Once you have done the above proceedure all the constraints are turned down to zero. to switch them on:
1. Press 'n' in the 3D View and the right hand panel should open up.
2. Second from the bottom is Rig Main Properties and when you select a control bone the IK influences for that bone are available as sliders
Well if that helped no one else it helped me :)
Next -> Weight painting (not as easy as it sounds)
How to weight paint a Rigify model
How to move a rigify model