Initially I thought about nice they would be if I printed them in color on the computer, used my paper cutter (which is packed away at the moment), laminated them, etc... and then I realized that a DONE project is better than a PERFECT project! So I whipped it out in just an hour or two and while it's not perfect, it's still effective and that's what matters.
I color coded mine in Do Re Mi colors since my kids are already familiar with the sequence and I figured it would help them remember the order better. The cards weren't long enough for some of the bigger numbers, so I wrote on them first and then used packing tape to tape them together. They are relatively sturdy.
I included a small chart to set next to it to simplify reading the columns. My kids knew some of these already, but not up to a million. We have a spiral flip chart where all of the numbers are already connected, but this way of layering them is just more child friendly IMO. They can see everything at once and grab whatever they need, versus only being able to see one number at a time on the chart.
I measured out two inch lines to create a guide, which I laid next to each card as I wrote on it. It's important that each column is covered up properly when layering the cards so the numbers needed to be sorta-kinda straight and evenly spaced. :)