From the opening picture it is rather hard to tell what the real reason for the defect leaves is. Watering not enough (especially in the growing seasons) will result in shriveling and dropped leaves at the lower sections of the branches. Over watering is really hard with the proper soil.
It is important to threat the plant as succulent. In the growing season one must water it properly and then let it almost dry out again before watering it again. Checking the weight of the pot with a kitchen scale usually helps to find out how dried out the pot is already until you get a feeling for the situation.
Depending on the region where you live the plant might require a dormancy phase during the winter (not enough sun), then keep it a bit cooler (between 10° and 15° C is good, IMHO) and water minimally (I usually only water once every 3 to 4 weeks on warmer days then, depending on the outside temperature.)
The soil should have a very good drain. Extra drainage —e.g. pebbles— at the pot's bottom is very useful too. I use a mineral mix out of pumice, diatomite, Akadama, gritty sand and only around 25% humous material (usually peat and coconut fiber). The structure of the mineral components depends on plant size; bigger plants get more coarse components.
While the plant requires a lot of light one should always be carefully placing it into full sun quickly after it was located in a more dark spot for some time; this will usually result in scorched leaves. Giving it some shading for a week or such until it has acclimated it a good idea there. Assuming it really is a variegated species then it might be generally more susceptible to this. A good airflow might help here too (e.g. with high temperatures in summer outside with wind is certainly better than behind glass w/o any airflow.)