Open the Photos app
Tap Select
Choose the photos you want to use
Tap the Share button
Tap Print
Use two fingers to zoom in on the preview until it opens as a PDF in Safari
Tap Share again
Save the PDF to Files or open it in a collage app
Use a collage app from the App Store if you want a true photo collage
Open the collage app
Select a collage layout
Add your photos
Adjust the size and placement
Save or export the finished collage
Share the collage to Photos, Messages, or social media