Going by bus would be awkward. You'd need to get a bus heading to Chum Thong from Chang Puak Bus Station, (see map, it's north of the square city moat) leaving every 20 minutes from about 5am to 6pm, taking 90 minutes. Then, get off at the sign to Doi Inthanon after 60 kms, just before the town. But then you'd be stuck! They'll be a tuk-tuk or songthaew that you could get to the entrance,,,then ..hitch! (pretty risk-free in somewhere like a national park...lots of families...).