The first step if you want to knit a mitered square blanket is to figure out how much yarn you have available for that blanket.
You’ll want to make sure that all the yarn you choose is about the same thickness. Your squares won’t line up size-wise unless you have consistent gauge across each of your squares.
Once you’ve gathered your yarn and decided on a needle size, knit yourself a mitered square.
You can make your squares larger or smaller by casting on more or fewer stitches. Just make sure that you cast on an odd number.
Different mitered square styles use different decreases in the center. Which one you use is really a stylistic preference, and you should feel free to choose the method that works best for you.