Step 1: Choose a Cage
Step 2: Find a Backpack
Step 3: Attach the Cage to the Backpack
I found loops on my backpack which looked designed for attaching things to the pack. So easy! No need to modify anything. I just used cable ties to attach the cage to the loops.
Step 4: Introduce the Birds (Gently!)
I gave the birds a gentle introduction to the cage, by feeding them in it over a few days, followed by some slow even walks around the house and yard they already knew. In the following days, I introduced the birds gradually to the world outside the front door, starting with very short walks and taking care not to freak them out.