Depending on how strong you are [and how good your scissors are] you can cut through from 2-4 slats at a time...instead of doing it one at a time Try it out first to see what works best for you. I found that 3 slats cut at one time was best~
Make the first cut on the hole where the string usually goes, as shown in the 1st photo. I find that this cut will make an average-sized stake, and the size plant marker I use the most.
Next, match that first set of cut pieces up to the rest of your blind slat stack. [2nd photo below] This will guide you so that your stakes come out roughly the same size, although they don't necessarily have be.