Open Assignment ready from the Home screen (or use its own page link).
Tap Choose files and select the photos (or PDF pages) of your assignment, in order.
Type your name and roll number (it is printed at the top of every page), keep Page numbers ticked, and choose the Maximum size: 2 MB, 5 MB or 10 MB, to match your portal.
Tap the big purple Start button and wait a few seconds.
When you see Done!, tap Download. The new file is saved on your phone or computer.