I've done both methods. I've photographed negatives on a light box. It is important to create a black mask so only the light coming through the negative reaches the lens. The room in which you are working should be kept as dark as possible. Also, depending on what lens you are using, you do not want to stop down to far. An aperture about half way works well. If you are using a lens designed for close focusing, then you can stop down further as a rule. Don't forget to calculate bellows extension factor and reciprocity failure if necessary. This will yield a film positive, unless you reversal process the film.

In the darkroom you will probably need a neutral density filter so that you can get exposures that are not too short for your timer. I cut small strips of film and then test, just like when making a print. Then that film positive will have to be printed again. Same as with the camera lens, don't stop down all the way unless you have a very good APO lens. Make sure your finished negative reads right emulsion down.

In both cases don't over develop. If you are new to this, keep notes, don't change more than one thing at a time, and be prepared to have to do it a few times to get it right. This sort of thing is a try and try again experiment. This how master printers become masterful. Do it until you get it right, keep notes and then you have mastered a new technique.

best, Doug