Position it where you want it and put a few or 4 screws into it from inside the cab. Clamps help as well as pre-drilling.

Not sure what you mean by "flush". An 1/8" back from the front of the face is fairly common as it's easiest to make it work all different door overlays. Anything other than full overlay would look kind of strange with a flush side panel.

With full overlay, I like the panel to come out from the cab a bit to sort of mimic an inset design when the door is closed.

You need a skin to flush out the frame reveal, or to have ordered a flush finished end. Then the applied panel is istalled over that.

Hi,
If the idea of a vermin-space behind the panel does not bother you, buy some 1/8" thick spacers (plumbing washers?) or shims and use them around the screws you'd use to secure the panel.
If the idea of a vermin-space does bother you, get some 3/16" plywood and shim the whole thing. A piece of paneling is going to be a whole lot cheaper than ordering a cabinet skin just to bury it again.
Casey