I have created a custom blog module that I want to use to store some aditional information about every blog post. I included this module in the header of the blog post page. Now let's say in the module I have a field called 'event_date'. I want to display this field from the module in my blog post page.
When you create a field in a custom module you can click on Copy > Copy Snippet to copy the field:
When you paste that snippet into the HTML + HubL section of your module and preview it, you should see the data you've input into that field in the preview:
Using that module in a template should display the same information that you can see in the preview.
Apologies for any confusion! If you add the module to a template and click into it, when you scroll down you should have the option to set information in the template:
At the page level you'll also have a similar option: clicking into the module will let you update it on the left-hand side (this doesn't apply for blogs since everything is done at the template level).