It really is a weird transition. For us, we had to cap night sleep to get the first nap, cap that nap in order to get the second, cap the second to get a decent BT. It was maddening. We did that from 12 mos-15 mos, then finally totally stopped giving the second nap and slowly pushed the first nap later and kept BT early for awhile. Once the nap got a bit later, BT got later as well.
After dropping the second nap at 15 mos it took him about 4-6 weeks to get used to it and the nws and ews stopped. We really had to resist the urge to give two naps and forge through it. It was tough but once he adjusted it was smooth sailing.
Also expect the nap to be shorter at first once you drop the 2nd. It's part of their body adjusting and settling into one nap, and it does get longer.
Good luck! You'll be on one nap before you know it!