Open Snapchat
Tap your profile icon in the top-left corner
Tap the gear icon to open Settings
Scroll to Memories
Tap Backup Progress
Turn on Smart Backup or Backup by Wi-Fi Only
Make sure you are connected to Wi-Fi if required
Open Memories by swiping up from the camera screen
Select the Camera Roll tab
Choose the photos or videos you want to back up
Tap the three-dot menu or Save option
Save the selected items to Memories
Wait for the backup to complete
Check Backup Progress again to confirm everything is backed up