This is what you need to know.
How to flash french door.
You can use a piece of aluminum roll stock if you.
Apply a bead of caulk to the felt and set the window in the caulk.
At the top tuck metal drip cap flashing under the siding and on top of the second layer of wrap.
Buy the corners here.
This is updated training on french door installation.
Install the drip cap under the house wrap and caulk all the critical areas.
In fact the absence of effective flashing at this 20 year old sliding glass door photo above led to the need for a complete door door jamb and trim replacement on this poughkeepsie home.
Next comes another layer of building wrap installed like the first layer.
This flashing will then be bent so it covers the top of the door trim and then bends downward again to cover the top half inch of the door trim.
Install the trim just under the drip cap.