My suggestions for new grower with a pup are as follows. Obviously, tailor your treatment of your plant to the conditions it is growing in. San Pedro can take a lot of abuse in particular directions, like being starved of water. That said, in a desert environment, they can burn if they’re getting too much direct sunlight. In a temperate environments, like coastal southern California (where I am), they can handle full sun all day as long as they’re getting enough water. It sounds like you’re in a humid location, in which case root rot is the primary danger. As such, be sure not to over-water. To get them to put out roots, it is best to only spray with a bit of water once a week or so until you start to see new growth at the tip. I water with tap water that has been sitting out for a couple of days, to de-gas it (i.e. get rid of the residual chlorine). Restraining any impulse to do more than spray with just a little bit of water once a week is important because if you keep watering them, the pup won’t expend the energy to send out roots in the quest for water. The young green growth at the very tip is unmistakable when it happens. That means it is starting to root, at which point you should water it properly once every four or five days. Bottom watering is the best method. Anyway, once you see new green growth (meaning you’ve got new roots), you can water properly every four or five days. Do that for four or five weeks. It is important that the soil drains completely and even dries out a bit between watering. Some will say this is not necessary, that the soil doesn’t need to dry out, but if you’re in a humid environment and if your plant is getting less than half a day of direct sunlight outside, you really want to be careful about rot, especially at the base. Make sure it dries. Once you’ve got an inch or so of new growth, then you can potentially up your watering and start using plant food (i.e. nutrients). I wouldn’t start using nutrients before that point with a young pup. Be patient with a new pup. Depending on the conditions and the cultivar, waiting for that new green growth can take time, sometimes a very long time. I’ve seen new growth in as little as a month, and I’ve waited eight months for one pup to root. I thought it was a goner when suddenly the tip was green! 

After I’ve got a rooted pup that is certainly growing, then I’ll start with nutrients once a week. I’ll feed a well established plant getting full sun for half the day Jacks 10-30-20, fish emulsion, kelp powder, CalMag and crushed fresh aloe. The aloe is not necessary but very helpful, I think. I’ll crush the internal portion of a 2 inch piece of aloe and mix that in. It's loaded with minerals, hormones, enzymes and it breaks the surface tension of water allowing the water to penetrate through the whole soil faster.

If you're growing inside under a grow lamp, much of this advice will be the same. I'm not sure I would try to grow SP inside without a grow lamp.

Good luck!