Installation with buildout
Add collective.taghelper to the eggs section of you buildout
eggs = collective.taghelper
Activate the product in your add ons section. This will install a Tag Helper controlpanel in you site setup.
In the Tag Helper controlpanel you have to fill in the API Keys for the webservices you want to use and choose if you want to use the local content or the destination of a link if your content type has an attribute remote_url (In Plone OOTB this would be the link type) and activate and arrange the tabs for the services. For the services that support this you can also define a relevance threshold so you get only the most relevant tags.
The product adds a new tab for your content named tagging (see screenshot). In this form you can choose the keywords you want to add to your content. Keywords assigned earlier manually will be preserved.