What are the best ways to care for my cacti?

The tilting one is getting too much water and may be dying. You should water cactus plants only when the pot, when lifted, feels empty and light. Keeping cactus plants damp is not correct. When they get dry, you water them well, and let them dry out again before adding any more water. Over time, the bottoms will get brown as they age, but a healthy plant stays hard at the bottom, when they get mushy, they are dying. Quit watering the leaning plant completely, be sure it's draining well, put in the sun and hope for a miracle.