How - General
Enter keywords into a text field offered by the site.
How - Specific
Del.icio.us - enter keywords separated by a space. This means that no multiple word tags are allowed. Use CamelCase or underscores (_) to separate words.
Flickr - enter keywords using quotation marks to connect phrases, eg. "New York City" would be one keyword. Multiple word tags are acceptable - this makes it harder to choose a single tag that everyone can agree on for a specific idea.