You're seeing lots of them because this week's CAS challenge is a sketch based on a card where Loll used the colour blocks.
To get the effect you need a piece of paper with the square (or whatver shape you want) die cut or punched out. Use the paper with the hole, not the punched shape, and sponge ink onto your card base through the hole - that's why it's a "reverse" mask as you'd normally mask with the shape itself, not with the "waste" from round it. For multiple squares, just move the "hole" and sponge again (start with your lightest ink colour and work to your darkest if you're going to do this - a little ink may smear off the edge of the hole and it won't matter if you work light to dark).
HTH!
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