Check them periodically, remove any that are becoming squishier -- those are spoiling, not ripening.
Each year it blooms and oranges start to emerge, but then the oranges fall off before they develop. We’ve given it Epson salt and fertilizer and moved it out of. Apr 03, Some oranges will stay on the tree before ripe but yes, as you said, they fall off the tree early. This is because the oarnge is ready, though a may be a different color let it sit out in a bowl on your kitchen table or on the counter, after about days it should ripen!
Oarnges fall off the tree early so they can ripen without the tree. They do continue to ripen. The skin will continue to develop color from green through mottled green-yellow on to the final color.
The scent and flavor of the skin and oils will change from a very"green", slightly bitter, citrusy smell and flavor (very generic, the lemons and the oranges at this stage smell and taste nearly the same) to a fruitier smell/flavor.