You might start by looking up portals in FileMaker help to make sure you understand how to set up the basic relationship that will control what records appear in your portal.
If you enter layout mode and double-click your portal, you can bring up portal setup.... In this dialog, you can specify that the portal display only one row. After making that change, you can resize the portal row to make it tall enough to suit your purposes. You can then position and resize fields with in this portal as needed to create a "miniature form" inside the form.
Whether you stick with the default "list" appearance or make changes to get your "form", the fields in the portal are just as editable as if you went to a separate layout for doing this. The only fields you won't be able to edit are calculation fields or fields where you've used the Inspector's behavior settings to deny access to the field.