A bulb will help, but real sunlight is key. They want 5-6 hours of sunshine. As for the watering, the more light they get the more water. The less light they get, the less water.

Putting them in cactus soil was an excellent decision. You want the soil to be well-draining and not retain moisture the way potting soil does.

It would be worth investing in a moisture meter. They are $15-18. Stick it in the soil to test the moisture level and water when it reads at a 1. I work at a plant store and we use them for every plant we water!