Some of the siding mfg's actually make flashing for heads, jambs & sills. Most do not.
What we normally see at the head is an 'L' shaped piece with a drip leg that sticks out over the window. The vertical leg is about 4 inches high with the horizontal leg being the thickness of the window the sticks out from the framing. The drip leg is 1/2 inch and at a 45 degree angle to the horizontal. The vertical leg would slip under the siding.
The jams normally use a 'J' channel similar to the ones used for vinyl siding, only with a deeper J. The siding will sit in the channel of the 'J'. Watch the layout so both jambs are in the flat portion of the siding.
Sills can be a real pain. If the window has a drip edge under the sill, great, just use a 'J' channel like you did on the jambs, making sure the window sticks out further than the J channel. Without the window drip edge, you will need a piece similar to the J channel, but the short leg of the J will have a drip leg like the head.
Clear as mud, I'm sure. Get to some of the metal siding mfg's websites, most will have cross-sections of their trim and flashing pieces.
Lastly, many of the mfg's sell a foam filler strip that matches their siding profile. These are for use at the head and sill. They come in both a front and back design so you can seal between the flashing and the siding if you cannot get flashing to match the siding.
Oh yeah, be very careful to get everything caulked and then, caulk it again. I hate water leaks!
Good Luck,
