It depends if you want droplets of water dispersed in the diesel (an water in oil emulsion - W/O) or droplets of diesel into the water (oil in water emulsion O/W). Whichever you choose the important thing is that the surfactant goes into the continuous phase. For example to make droplets of diesel in water you should dissolve the tween in the water (start at 1Wt%) and then mix in the oil.

For the W/O put 1% span into the oil then mix in the desired amount of water. In each case you might need more surfactant depending on how much dispersed phase you want to add. I hope this helps.

If we speak about microemulsions there is  n o  dispersed phase. Microemulsions are micellar solutions!