Succulent plants store a lot of water in their tissues, so they are very resistant to drought, because when rainfall is scarce or absent they re-use the water in their tissues. Therefore, it is not true that succulent plants must not be watered, otherwise they are no longer able to supply their internal reserves and they will mummify.
This type of plant only needs watering if the climate is hot and dry, so avoid watering them during the winter, but from the spring until September remember to provide water from time to time.
Water only when the soil is really dry, leaving it to dry for a few days before getting it wet.
When going to water the succulents, take into account the fact that they are often potted in soil that is very well drained and dry. You will have to devote a good deal of time to moistening them completely. So water rarely, but with a good amount of water that will thoroughly wet all the soil contained in the pot.
If the soil is very dry and very sandy it may happen that the water you supply drains completely from the ground. So it will be necessary to keep going, providing water for a good period of time.
If the ball of soil around the roots is dried and hardened, it might be useful to water your succulent by immersing the vessel in a container filled with water, so that, like a sponge, the soil absorb all the water that it requires for ideal dampening.