It will require two different set ups of the fence. To make the vertical plunge cut, place the fence registration adjuster in the longer curved slot at 0 degrees. Mark out a center line on the top of the T and mark where you want the centers of the biscuits, hold the leg of the T on the top and transfer the biscuit center marks before cutting. Clamp a straight edge to the work to serve as a guide, set so the joiner against it is centered on the line. Use the center marks on the joiner to align with the biscuit marks. The joiner will be straight up and down, perpendicular to the work. The sandpaper face provides grip but still go slow so the joiner doesn't walk.
To cut the slots on the leg of the T, set the fence for a 90 degree cut as normal, centered in the thickness of the work piece.