I have not done it since the 1950s, but I used a weak green bulb which I kept a few feet away from the film. The idea is you half-develop the film (or, what you think is proper amount of time for half development) and then take the film out of the "soup" and look at it briefly. It will look muddy, like film that has not been fixed properly. The idea is to look for the dark areas in the film, the developed areas. It is a steep learning curve and you have to practice. But you will finally learn how much more development is needed. Or perhaps that you have to stop the development at once, that you have developed it sufficiently already. Or, you may decide you need to put it back in for several minutes or even switch to a more powerful developer because it is very underdeveloped. Good luck. I am putting together a FrankenGraphic (from spare parts) and will hopefully be shooting 4x5 soon, again.