How do I display specific custom posts, and how do I edit a post’s singular page?

Custom post types don’t need a template created for them. They will automatically use the archive.php and single.php of your theme. If you want to customize a page so that your portfolio and your regular blogs are different then you need to create 2 files.

archive-postname.php (in this case it’s whatever you named your post type ie “portfolios”) This loads the archive page of your cpts or the roll, basically the list of all your posts on one page. You would get to it by going to www.example.com/portfolios/ (or whatever you’ve set your posttype to be)

then you have your single-postname.php

this loads the post detail or single view. this is what someone would see if they went to www.example.com/portfolios/cool-project-1

This is based on a few things. I’m assuming your post type is exactly portfolios, and that you don’t have an slug redirects etc. It’s also assuming that in settings/permalinks you have set to %postname%.