Creating new post meta items?

Hello, I am building a site based on Templatics Real Estate theme.

Anyhow, I want to create a new meta key, or custom field. Im not really sure which one.

The theme has items for properties such as baths and bedrooms, they are referred to in the code in the following way:
$proprty_bathroom = $_SESSION[‘property_info’][‘proprty_bathroom’];

This is just an example.

Anyhow, I want to create a new one of these items, but I am unclear on how? Where might these items be being created?