I posted about it on Instagram, and someone asked how to make tablet weaving cards from playing cards, so I made a quick photo set to illustrate.
First you have to square up the cards. I overlap them at right angles, making sure two sides line up, and draw a line.
Cut on the line, and round the corners. Some people purchase a corner rounding punch - apparently there are such things, among scrapbooking supplies at craft stores. I saw some and could not figure out which corner rounder I wanted - they do cut slightly different shapes - so I just judge it by eye and do it by hand with my scissors.
I then use a ruler and mark a set of lines half an inch in from each corner. This is where I will punch the holes.
I punch the holes where the corners are marked.
After that I mark the A-B-C-D on each corner with a Sharpie marker. Some people don't mark the A-B-C-D, but I find it makes it easier for me to follow where I need to stop and start in a pattern, as well as letting me know where I stopped. When I'm doing tablet weaving, I try to stop after one full repeat - so if I start with the A at the top away from me, I want to do my 4 forward, 4 back, and end with the A in the same place. I suppose the card number could serve the same purpose, but it doesn't take me long to write on the cards.
Someone else said she colors the edges to help her keep track in a similar manner.