The day after we brought Molo home, I began to leave him on his own in his crate, cos I knew I only had a few weeks to get him used to it!
The first day, I tired him out by playing with him, made sure he was toileted and then lured him into his crate with a treat (later a stuffed kong), shut the door and left the room. I immediately came back into the room (before he had a chance to whine, bark, scratch or get upset) and let him out of the crate again.
I did this over and over again; many times an hour, very slowly increasing the time from a second, to 10 seconds, to 30 seconds, to a minute......sometimes going back in and not opening the crate, sometimes leaving again, sometimes letting him out after making a cup of tea...just getting him used to be alone and reassuring him that I always come back
After a couple of weeks, he was happy to be left while I popped to the shops; he got used to the signals that we were going to leave him, and was happy to be left because he knew what to expect
When Bonnee arrived at 15 months old, she suffered from mild SA due to the change of home, and she cried every time I left the room
I used exactly the same method and after a couple of weeks, she was fine being left, too