Hmm - my two table builds are not exactly great for instruction but you may glean something (the links are below). It’s a bit of a reminder - I’ll try to do better next time!
Don’t set up on the floor!
Get two trestles or a table or anything that you can put two “beams” exactly level. By level, I mean sight along the tops and make sure they are exactly the same - you can do this better by eye than by measuring. If you don’t understand the above let me know and I’ll draw a picture.
Now secure the beams using clamps or better still screws if you have a sacrificial surface.
Place the torsion frame spanning the beams - check for square and use chocks on the beams or even a bit of string if have to on the diagonal to pull it square. If the warp is still apparent, use clamps to hold it to the surface of the beams - it should then be level, square and flat.
You might have to use some cleats on the frame to hold the clamps so you can skin the top.
Now fit the top skin. (or the bottom one if you love working upside down)
When the glue has dried you should have a “less warped” top - flip it over fit the remaining skin and clamp it into position.
It’s not as hard as it sounds, and you should end up with a really flat surface unless you have an alignment problem with your joints - best to check them before you start.
Not sure if the links below are helpful, or even entertaining to anyone but me.