How to generate a high or low?
You connect those signals to Ground (0) or Vdd (1).

The purpose of those gates in the circuit is to act like an inverter. You could replace both of those gates (the NAND and the NOR) with an inverter each and it would act the same. The fact that you have inverters in the schematic makes me question why they're not inverters.

If this block is going to be black-boxed and used in a bigger piece, I'm surprised those lines aren't connected to input lines. In that way, you could have them act as inverters in one block and normal gates in another usage of the black-box.