Here is how you make a stick shifter from an ordinary grip shifter.
You can use a bolt-on downtube shifter from an old road bike for a bracket.
You also need a bolt that will thread into that shifter bracket.
Cut ¾ inch or so from the end of an old handle bar. Aluminum is better.
Use the wedge nut from the bottom of a threaded handle bar stem on the end of the handlebar piece.
The wedge nut will connect the handler bar piece to the downtube shifter bracket but still allow the grip shifter to slide over.
Run the bolt thru the wedge nut, through the handle bar piece, and thread into the shifter bracket.
Cut a piece of 5/16 inch threaded rod for your shift lever.
I used the curved cable hardware from a side pull mountain bike brake (the macaroni?) between the shifter and cable housing. The cable must take a 90 deg curve after exiting the shifter, towards the rear of the bike.
I drilled 2 holes, 180 deg apart, through the rotating part of the gripshift to attach the shift lever.
Drill the hole slightly smaller than the threaded rod.
Mount the shifter to the bike frame and attach the shifter cable before drilling.
You want the shift lever to be vertical when you are in your cruising gear (in general).
Spin the threaded rod into the holes in grip shifter. Secure the threaded rod with a nut on top and bottom of the grip shifter.
Slide on the rubber boot that came with grip shifter back on. Trim it around the nuts.
Cover the open end of the grip shift with a bottle cap.
I covered my threaded rod with clear plastice 5/16 inch tubing. It will make it look thicker and resist
staining and corrosion,
Top off your shift lever with a wooden knob from a craft store. Drill a hole and thread it onto the threaded rod. Paint the knob your favorite color. Pics below.