Answered on FB by the esteem’d jon Webb, and copied to here:
You would pick a unit to throw one or more grenades, then declare a point on the table they are throwing at, in range and LOS as usual.
You then scatter the template d6" as noted. If you have any models under the new position (possible on a low roll against a large unit) then you roll damage.
Every grenade after the first has you place another template touching the first, so you could chain 2-3 grenades to fall back to the initial targeting spot if required.
The first one is fairly inaccurate for sure, but throw enough grenades or target a large unit and you can make a mess.
The scatter direction is 2d6 combined, so you can also be a little clever there as you know 7 is the most likely direction it will scatter, so you can use median averages to have a good idea of where your template will scatter too, and use that to position accordingly.