VNAV is a bit complex and has different functionality, depending on FMC and phase of flight. For example, if your lower target altitude is at a waypoint that is still in the cruise portion of the FMC flight-plan, you may not be able to enter it on the legs page without an “S” after the numbers. For example 310S which means a step-change in the cruise altitude.
Another way of programming a VNAV descent is to put a lower cruise altitude in the VNAV cruise page and then push the MCP ALT selector.
Different models of 737 and different upgrade status of the FMC will have different capabilities, so it’s not easy to give a short answer. The main point is to make your inputs and then MONITOR the FMA to see what the aircraft is doing. For VNAV, one crucial aspect is to understand the relationship between the elevators and the auto throttle. Which of these is controlling speed? Are the autothrottles active or just armed, or in HOLD?
Good luck!