The easiest way and depending on the season, to get to The Niseko resort is by private transfer. A more affordable option is scheduled coach which run frequently in the winter but less so in the summer.
For those with experience in snow, renting car is also an option however be sure to ask whether parking is available at your accommodation.
If you are taking the train, you will need to change trains at either Otaru or Sapporo Station and ensure that you get off at Kutchan Station and not Hirafu Station.
If you are staying in Hirafu, most of accommodation is within walking distance to lifts, restaurants bars etc. and you can jump on a ski shuttle to get you t and from the lifts.
Taxis are available to take you to Kutchan or other resorts but you can expect long waiting times in peak season.
If planning to come during peak season it is advisable to book any services and activities before you arrive as they tend to fill up.
Lift tickets can be pre ordered and delivered to your accommodation for convenience however you can book online and pick up at the lifts on your first day of skiing and save.