This would need 2/3 support in both houses as Trump presumably would veto the bill. Granted, that's the same threshold anyway for upholding a challenge to being disposed by the cabinet.
Would a measure declaring the president incapacitated have to be approved by the president? It seems to me that the resolution would be delivered to the President Pro Tem of the Senate.
The move is to pass a bill to give Congress the power to do this. (They don't currently have the power to say he's incapacitated, just to specify who can say he is.) So it has to pass the same as any other bill. Either he signs/ignores it, or it passes over his veto.
