What do you input in a navlog for wind direction and velocity, when the wind and temp forecast says 9900+18? I know 9900 means light and variable winds less than 5kts but how do I determine the average wind direction to use for my navlog?
Light and variable means there's no meaningful windspeed or direction, so you can just ignore it. For planning purposes, you can simply plan as if the wind was calm.
But, it's very possible that the winds will actually be stronger and more directional than forecast. You should check your actual track and times against what you planned, and adjust your navlog in flight as needed. That's one of the cross-country navigation skills that pilots are expected to learn.
The best technique when planning your flight is to assume a 15knot headwind ALL the time. This allows for climbs, descents, and delays, and works quite nicely.