Cut out some frames. If you are using actual film and choose to edit it on the negative itself, you need to cut out every few frames. Cut somewhere between every 10th to 15th frame. When played back, this lack of a few random frames mimics the flicker.
Use a post-production program. If you are using video or digital, continue filming as usual. In post production, there are programs that pull frames out to your specifications. You simply choose the frames you want removed and click. When played back after this procedure, it simulates an old-style flicker. You can also add scratches and other irregularities to make a movie look old.
Do not direct an actor or have anything going on in the shot be adjusted to try to mimic the stutter. This won't work and simply looks silly, rather than duplicating the flicker of an antique film.