I created this card for one of my mum’s colleagues. It is an original design cut out of black paper. I placed a shiny textured paper behind it which makes the black papercut stand out. The shiny paper is really cool because it changes colour depending on how the light is reflecting off it. I then taped (using double-sided tape) another piece of black paper onto the inside to cover up the coloured paper.
If you’re looking at this because you want to make a card that you’ll have to place another piece of paper inside of, I have some advice: don’t cut the paper first. Tape/glue a piece of paper larger than the card in and then cut it. This will stop you stressing about it being straight and also decrease the likelihood of stuffing it up. (Hopefully that makes sense.)