WordPress.org

Download TwitterCounter.

Extract the contents of twittercounter.zip to wp-content/plugins/ folder. You should get a folder called twittercounter.

Activate the Plugin in WP-Admin.

Goto Settings > TwitterCounter and enter your Twitter username

Add <?php do_action('echo_tc'); ?> to your theme file where you want to display the counter or use the WordPress widget

Add <?php do_action('echo_twitter_remote'); ?> to your theme file where you want to display the counter or use the WordPress widget