>>2801698
A) Put the backpack on sideways, so the backpack's straps are holding onto the suitcase's handle.

B) Use an old belt to secure your backpack to your suitcase.

C) Buy a luggage strap (pic related). No need to make it too complicated, there's no recommended brand or anything like that. They're so ubiquitous that you can even find them in dollar stores. Same method as B, use it to secure your backpack to the suitcase.

D) Wear the backpack.