FlyerTalk Forums Expert How-Tos and Guides
 

7/2/2018 update (either the link or this method may die at any point):
Here's the steps for those booked Asiana flight using UA (or other) miles.
For all award trips, "Find my trip" doesn't work.
Select "Check-in" and fill the information. But not through the check-in button at the bottom of the first page. Go through "BOOK" at the top bar and then click "check-in". https://flyasiana.com/I/US/EN/CheckIn.do#none
You must have the ASIANA record locator number, which is available on the United site for your reservation.
It won't allow check-in because your travel date is too far away. But it lets us to select seats !!
Remember: between 72 and 48 hours before each flight, all good seats (front Y) are open.

======
Note: Seat selection now available at time of booking, before payment.

Online Advance Seat Reservation
Go to Reservations > Click on My Reservations > Login or Continue as Guest > Find or Enter Reservation Number > Click on Advance Seat Reservation located near the bottom of View Reservation Info screen