Interpolate vertically within the relevant boxes and then horizontally between these adjacent boxes.
Same for temperature.
Just because you are a student on a qualifying cross-country doesn't absolve you from responsibility nor should you consider yourself any less affected by the weather!!!
Shame you haven't been taught this before today.
So, for example:
50N 05W
05 050 25 +07
02 040 15 +13
50N 0230W
05 040 25 +09
02 040 20 +13
For 3000ft (vertically) (say, in round figures):
50N 05W
03 043 18 +11
50N 0230W
03 040 22 +12
So if you're between the two points (horizontally):
03 040 20 +11
Nearest 5 degrees of wind direction is fine;
Nearest 5 knots of wind speed is fine;
temperature is above freezing (main point);
So, in this example: 03 040 20 and no known icing would be acceptable.
In more severe weather the gradients will be greater but the above will hold true in most cases.