Changing photo orientation

The Print dialog is taking care of the orientation.

When you select the photo in Photos and then enter ⌘P to print it, you will be seeing several templates, in landscape or portrait orientation. Pick the one you like best, You can also specify the aspect ratio you want, if you select a custom template.

In the Custom Template you can specify the width and height you want.

You need to crop it to the size ratio of the landscape print you want to create. From experience you probably only be able to include a small part of the portrait image, i.e. just their heads and shoulders or similar.