Thanks for your guys' replies. Here is a picture of it. (Couldn't fit it all though) The cabinets/boxes are made of 1/2" MDF.
So I should definitely pre-assemble some sort of face frame rather than nail the moulding/trim up one piece at a time?
It looks like you've already installed all the boxes. At this point, one of the choices (and maybe the easiest) would be to use a hand block sander (sandpaper wrapped around a block of wood) and flatten the joining cabinet ends and leading edges all around. Then contact cement the pieces to the front edges.
If you have flat edges to start with, solvent based contact cement will hold very well. Start at one end and just put them on like an ongoing puzzle.
If you prefer to use yellow glue, you can drive into the front edges 4d finish nails (keep them straight), and snip off the length on a sharp angle to protrude about 1/4". Then, apply a thin layer of glue and use a block and hammer and tap the FF pieces onto the front edges.
Or, if you're painting, just nail them on to the front edges, and seat the head with a nail set, and fill and sand. You could do this with a finished wood (natural or stained) FF and use a matched fill. I would not use pocket screws or biscuits. If you need to tie the rails to the edges of the stiles, drill a small pilot hole on an angle from either the top or bottom or both of the rail into the stile, to drive in a finish nail. Seat the head with a nail set.
Making individual FF's or large sections of FF would also work, but would be more labor intensive. If you choose to go that way, HERE
is a thread that might be helpful.