It's the same as 1D interpolation except you repeat it a few times. So, if you have data at 0,0 0,1 1,0 and at 1,1 and you wanted to get (0.7,0.3), you could start with finding the values at 0.7,0 0.7,1 and then interpolate between those two values.
https://en.wikipedia.org/wiki/Bilinear_interpolation
(see repeated linear interpolation)