Congrats on you machine.
To make a flat true frame or adjust frame/rail twist.
You need a straight edge that is long enough to reach rail corner to the opposite rail corner.
Use a level to find the elevation between the rails ,, (found across the rails closest point). Level the level/rails (while its across the rails closest point).
Make a point there. This point is just a spot to set the straight edge on.
Put the straight edge on that elevation point. Use the straight edge to make the center and the 4 rail corners "on the same plane".
Thats "in theory just the corners", If your rails flex/bend then you'll have to level there whole length.
Odds are when you get near the final level, the center point will have to be adjusted by a few thousandths, then continue leveling each corner.
Keep in mind- move the machine and it may be twisted again.
no cnc yet.