This article assumes that you have created a schema. If you haven't created one you should do that first.
To begin adding values to your metafields start by clicking the Edit Metafields button in the top navigation menu.
Select the object type you want to add a metafield to from the dropdown.
Click the Show Results button and your objects will be returned below with the fields you have setup in the schema for this object type.
From here you can edit the field you need to and then click the Save button.
Success! You've just added a metafield to your object.
For editing metafields in bulk please see