If our table looks like our news table:

CREATE TABLE newsitem (
    newsid INT PRIMARY KEY,
    newstitle VARCHAR(255),
    newscontent TEXT,
    authorid INT,
    newsdate TIMESTAMP
);

First we need to add a new column to store the full text index:

ALTER TABLE newsitem ADD COLUMN newscontent_fti tsvector;

If we want to index just the content field, we run this command:

UPDATE newsitem SET newscontent_fti=to_tsvector('default', newscontent);

When that finishes, we can index it:

CREATE INDEX newscontent_fti_idx ON newsitem USING gist(newscontent_fti);

When that's finished, we need to update postgresql statistics:

ANALYZE;

This helps postgresql decide when to use an index and when it's better to ignore it, so it's a very important step to remember.

This will go through each table and update statistics for it, if you want to do just a specific table you can do:

ANALYZE tablename;