My process:
Get two panes of glass or smooth tile, larger than the record. Preheat them to about 150 F, in any case don’t exceed 175F. Using oven mitts, place one glass pane/tile on a heatproof surface, put the cleaned bare record on it, place the other atop it.
Let it sit to cool with something about 5 pounds atop it. For a few days minimum, a week is better. You can move it to a more convenient place once it cools, about a half hour.
That will remove some warp. Repeat as needed.
Note that the center of the record is thicker than the grooved part, so not all warp can be removed, but it can get to playable easily enough.
Also, be careful to not slide the glass or tiles across the record! Also, you can use a hair dryer to heat things up instead of the oven.
The Google knows other ways, but generally you flatten it with pressure and time, maybe a little warmth.