Here's a rather two-faced way to perfectly align a drawer face prior to screwing it onto the drawer. First I support and position the bottommost face on shims before reaching into the drawer and pushing it against the face and into two pieces of fabric mesh, double-stick tape stuck to the left and right side of the drawer. Next I use this bottom face as a base, placing the remaining faces on top of it, equalizing all the spaces with a combination of different thicknesses of cardboard. The subsequent drawer is installed on its tracks, and it's face placed on top of the cardboard spacers, as shown askew, positioned laterally, and then pressed into the tape. The tape holds the face in place while the drawer is screwed into its back, which can be done either inside the carcase or with the drawer removed. Clamps can augment the process. If it's not possible to push the drawer forward I place foam behind it, just enough the make the drawer protrude slightly and provide compression for the tape bond. The tape is left in place and provides some pretty good adhesion, though the face can still be removed. It's a case where a stuck drawer is just what you want.
