Open the Photos app
Select the photos you want to use
Tap Share
Choose a collage app or shortcut if available
If using the Shortcuts app, open it
Tap the plus icon to create a new shortcut
Add the action for making a photo collage
Select the photos
Run the shortcut
Save the finished collage to Photos
If you do not have a collage shortcut, download a collage app from the App Store
Open the collage app
Import your photos
Choose a layout
Edit the collage if needed
Save or share the collage