Use a balanced, low concentration fertilizer at each watering.
If the soil mix is well drained, Haworthias can be watered weekly in the summer months; less frequently in the winter. The soil should dry between watering. Prolonged wet soil, especially when it is cool, there is low light intensity, and/or the plants are not actively growing, can lead to root rot.
East or West facing window where they get a few hours of direct light daily, and bright indirect light through the remainder of the daylight hours