In it’s simplest implementation, you basically create a TV to hold the meta info you require.
Create a TV (for this example it’s named "Meta_Keywords")
Input type: Text (or Textarea)
Default Value:(comma seperated list of your keywords used as default) Stuff,More Stuff,Stuff2
Make sure you check the templates which this TV is allowed access to.
Add this to the top of your template: <meta name="keywords" content="[*Meta_Keywords*]"/>
Now for each document which uses the template you checked, there will be a text input TV named Meta_Keywords (which will be filled with the default content if you provided any) which can be edited on a per document basis.
You can get alot fancier, but this should give you a start point