Use two clean 330 x 330 mm ceramic floor tiles, each with a smooth top side.
Preheat oven to 120-160 degrees Fahrenheit (say 60-65C).
Place both tiles in the oven, face up, on separate shelves so they heat through thoroughly. Leave to heat until the thermostat is steady at 60-65C.
Using a protective mitten, remove the bottom tile first and place face up on a heat resistant surface.
Now place the record onto the tile (in a clean paper inner sleeve) and put the second tile face down on top. The smooth side of each tile should be touching the record. The weight of the top tile will be sufficient.
Leave until completely cool – at least an hour.
On removing the record, it will be flat, and undamaged.
Another method I’ve read about is to heat at 70C in the oven for 30 minutes (this is a 180g disc – probably need to reduce for lighter/thinner disc)
Remove from oven and place all under a large heavy book to cool for at least 2 hours.
A third method recommends 80C for 10 minutes, then cool – I haven’t tried this!
WARNING – some have suggested that these heat and press methods can flatten grooves.
Be very careful with the oven setting. I literally melted an LP by overheating!
I tried two glass plates in the sun – it didn’t work.