How to trim indoor ficus tree?

Healthy ficus withstand pruning very well, so if it is truly out of control you could cut it back fairly hard and then regular pruning can be used to keep in in the size range you need from then on. If you make a few tentative trims and watch the resulting growth pattern afterwards you will see how best to trim it in terms of individual cuts.

Overall it is better to do any extensive major pruning just before the big spring growth spurt because the new foliage hides the cuts more quickly. Keep in mind, too, that for a while after and extensive trim the tree's watering needs will be somewhat reduced to compensate for the lack of foliage.