When I changed my DS (13.5 mths) to 1 sleep, I dropped his morning sleep and then put him to bed after lunch at 12:30pm. He will usually sleep for a couple of hours. We had a few days of whinging at morning sleep time so we would go out for the morning to keep him occupied even if it was just to the park or the supermarket or out into the back yard. He soon got used to the new routine.
I knew he was ready to go to one sleep because he would take ages to settle for his arvo sleep, when before he would just go down easily. I chose arvo sleeps to coincide with DD's (2.5 yrs) arvo sleep. This way I at least get 1 hour of piece to myself a day.