To make a simple, paint-ready, Shaker-style cabinet door here is one technique.

Use a piece of 1/2″ MDF slightly larger than your finished sized for a base and stiles and rails of 1/4″ MDF.

Using glue and 23 gauge pins, the rails and stiles are attached to the base. One side should be flush, and the remaining pieces square to this side.

Using the flush side as a starting point, trim off the excess with a panel sled on the table saw.

Of course a door made this way is not meant to be part of fine furniture. But where a sturdy and simple door is needed it works fine.