I like to use really thin cardboard, like cereal boxes, because they're a decent size for artwork, sturdy, and smooth. They're also great for layering with other pieces if you decide to do more of a collage.

Other thin cardboards that would work just as well: cracker boxes and other snack boxes or even some frozen food boxes (we have a frozen waffle box that we used, along with some applesauce pouch boxes). Just use whatever you have. You could even use large packing boxes, like what you'll get in the mail from Amazon and other deliveries. But that type of (thicker) cardboard often has visible ridges, which I don't love.

Cut the cardboard down to whatever size you want your artwork to be. I kept some full size and cut others down to be much smaller, so I could collage them all together.

I usually don't use the sides of the box bc they're often too small. So I just cut up the front and back and recycle the rest.

Use whatever craft supplies you have on hand to create some simple abstract shapes. You can search 'abstract shapes' on Pinterest or Google to get ideas. Or even better, search 'aerial landscapes' and find inspo through the unique shapes found in nature. Sounds cheesy, and it is, but you can pull out some really interesting shapes from looking at aerial landscape photos.

You can also get some cool 'abstract' shapes by drawing letters in a more stylized way. For example, I made some 'T' shapes that kind of look like mushrooms and there are some upside-down 'M' and 'N' shapes too.

Back to the coloring / painting materials... I used tempera paint sticks from my son's craft cubby for all of this art pieces and I love how you can build them up with a few layers. I stuck with just a handful of colors, so that all of the pieces would feel cohesive. But again, since this is zero dollar DIY, use whatever you have. This could be markers, crayons, some leftover wall paint you had from a room makeover project, etc.

You can stop here and tape the pieces up on you wall straight away. I like to use blue painters tape and roll the tape into loops on the back so you can't see how it's attached and it doesn't damage the walls.

Or opt to collage some of the smaller pieces together to make one bigger piece. Or even add a frame, which you can make yourself, reuse one you already have, or buy something new. But since this is zero dollar DIY, I left everything frameless, at least for now. I do have scrap wood in the garage that would technically fit the bill for keep the cost at zero dollars. Maybe that's a project for next time.