When the winds aloft are light and variable, what wind direction and speed should I use for flight planning?
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.