How do I transition from one state to another with d-flip-flops? (digital lock)
Your combinatorial circuit has Q0 and Q1 inputs, representing the current state, and NEXT0 and NEXT1 outputs representing what the next state should be. 

It is the role of the FFs to hold the current state, so their outputs should be connected to Q0 and Q1.

You want the next state to become the current state on the next clock edge, so connect NEXT0 and NEXT1 to the inputs of the FFs.