From JR Asahikawa Station, take bus no. 39 to Shirogane Hot Spring at bus stop no. 8. No need to reserve tickets in advance. Just pay on the bus upon getting off.
Fare is 1070 yen if I remember it correctly, one way, getting off at the blue pond bus stop.
If you want to catch the return bus to Asahikawa or Biei, you must explore the pond in 30 minutes or less so that you would have time to walk back to the road to catch the bus (on the other side of the road). Otherwise, you would have to wait for more than 3 hours for the next bus. If you have your own car, then this won't be an issue.
I opted to explore the pond longer and instead of catching the return bus or waiting for 3 hours or so for the next one, I walked for about 40 minutes to check out the Shirahige falls in the Shirogane Hot Spring area where a few onsen hotels and tourist information center are located. Unfortunately, that day was a public holiday hence the information center was closed. It was cold and freezing since it was snowing. There is a hotel next to it where I sought warmth instead.
I took the return bus at the hot spring area to JR Asahikawa for 1180 or 1190 yen, I think.