I make face frames to fit flush to the outside of the carcass. If you have built it square, the faceframe should fit exactly. If over slightly, you can use a trim router to make it even. I generally make faceframes using 1.5 inch wide stock. Cut precisely on the ends, and cut all same length pieces at the same time or use a stop block. I put them together with pocket hole screws and the jigs cause the ends to close up tight and square (if the ends are square) Square is the key word. How you attach them to the carcass is a choice to consider, biscuits work and are invisible, but I generally just use glue or pocket holes inside and out of sight. Pocket holes are a pain to make disappear if they are visible. If you're using a table saw, get your Wixey out and make certain the blade is 90 to the table, and use a good square to make certain your miter gauge is 90 to the blade,or use a sled you know is 90 tothe blade.
I measure the face carefully then calculate the rail length, minus two style widths for an exact fit. There are lots of ways to do a faceframe, but pocket screws have made it easy and reliable for me, and they don't show from the front. Glue if you like but glue squeeze out can mess up a finish, and once attached to the carcass, the pocket screws are never going to let go.
The more I do, the less I accomplish.