Question in title.
But books lots of books works better
Put the card in a sleeve, then get a large book like a Bible. Open the Bible half way and put the sleeved card in between 2 pages, close the Bible. Check the progress every day or so.
In between two bricks
If you do decide to try to flatten, MAKE SURE your card is in between to perfectly flat and smooth surfaces (like halfway inside a Bible.) If you were to put your card on a rough or porous surface, then put weight on your card, it could damage your card and leave indents from the rough surfaces.
Pro tip, What I do: Between Two 24”x24” 3/4” thick glass for 7+ days.
Flattening can help. Use any of the serious methods above and apply some good old weight. Stack of heavy books on the bible would do fine. & days and that’s about all you can ask for.
I will try these. Thank you all very much.
The most effective method in my experience is placing the sleeved card under an internet modem for 12-48 hours. The low heat will slowly uncurl the card. After the card is sufficiently flat, immediately place it in a card saver or toploader and leave it for a few more days. Skipping this last part can cause the card to slowly regress to its original warped state.
That being said, some cards are beyond saving. I’ve found the foils from EX Deoxys and EX Team Rocket Returns to be the most difficult to correct.