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1. Go to your WordPress admin dashboard -> Plugins -> Add new, then search for Cat + Tag Fliter and agree to install it.
2. Go to youк widget options and change the widgets settings.
3. Sometimes it is needed to manually flush rewrite rules. Go to your Permalinks Options and change it to something else, save, and then change it back to normal.

If it didn't work, try this:

1. Upload cat-tag-filter folder to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Go to your widgets settings and add Cat + Tag Filter widget to your sidebar.

If your theme doesn't support widgets, you can use this code:

<?php the_widget('cat_tag_filter', $instance, $args); ?>

Here is a full list of default $instance arguments:

'title' => 'Filter'
'button_title' => 'Show posts'
'cat_list_label' => 'Show posts from:'
'tag_list_label' => 'With tag:' 
'all_cats_text' => 'Any category'
'all_tags_text' => 'Any tag'
'cats_count' => 1 
'tags_count' => 0 
'tag_logic' => 1 // 1 for AND and 0 for OR logic operator 
'tag_type' => 1 // 1 for checkboxes and 0 for dropdown 
'clude_tags' => 'exclude' //can be 'include' or 'exclude'
'exclude_tags' => '' //used for including tags also, if clude_tags is set to 'include'
'clude_cats' => 'exclude' //can be 'include' or 'exclude'
'exclude_cats' => '' //used for including categories also, if clude_cats is set to 'include'
'tag_tax' => 'post_tag', 
'category_tax' => 'category'

If you want to override some settings, for example get rid of title and turn on the counter for tag list use this code:

<?php the_widget('cat_tag_filter','title=&tags_count=1'); ?>

If you want to get rid of div's that WordPress creates before and after all the widgets, use this code:

<?php the_widget('cat_tag_filter','title=','before_widget=&after_widget='); ?>

You can also override before_title and after_title the same way.