On the Mobile App:

Step 1: Tap the "Account" icon

Step 2: select "Your Orders

Step 3: Select the order you'd like to view

Step 4: Select the view order details

Step 5: Here is your order number

On a Computer: 

Step 1: Tap the "Account" icon

Step 1: What is outlined in the box is the order and the order number that goes with it.