How to Use Undefined Constant Descriptions in WordPress

Using Yoast WordPress SEO
1. Access your WordPress dashboard. Click on 'Plugins,' and then 'Add New.' Search for 'Yoast SEO.' Click on 'Install,' and then click on 'Activate.'
2. Click on 'SEO' on the bottom of the sidebar menu. Click on 'Titles.'

3. Scroll down to the 'Post' box. Type the following code in the 'Description' box:%%excerpt%%
4. Click on 'Save Title Settings.'
Using All-in-One SEO
5. Sign in to your site. Click on 'Plugins,' and then 'Add New.' Search for 'All in One SEO.' Click on 'Install,' and then click 'Activate.'
6. Click 'Settings,' and then 'All in One SEO' on the sidebar.
7. Scroll down to 'Autogenerate Descriptions.' Check the box next to the option.
8. Click on 'Update Options.'
Using SEO Ultimate
9. Log in to your WordPress site. Click on 'Plugins.' Click on 'Add New.' Search for 'SEO Ultimate.' Click on 'Install,' and then click 'Activate.'
10. Click on 'SEO' on the dashboard sidebar. Click on 'Meta Descriptions.'
11. Type the following code in the 'Post Description' box:{excerpt}
12. Click on 'Save Options.'