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I made a mistake. I meant to say to make a VERTICAL slit, not diagonal. Just find a corner and slit 1/2" into the corner , so straight down.
I had the same problem, especially when the cut letters are so small. I solved the problem by turning over the cutout letter, taking an exacto knife and making a half inch diagonal slit into one corner. You don't want to go all the way through, so you need not press hard. Then I slid the exacto point under one side of the slit. It raises that part and you can peel off the back.